selftests/net: change shebang to bash to support "source"

The patch set [1] added a general lib.sh in net selftests, and converted
several test scripts to source the lib.sh.

unicast_extensions.sh (converted in [1]) and pmtu.sh (converted in [2])
have a /bin/sh shebang which may point to various shells in different
distributions, but "source" is only available in some of them. For
example, "source" is a built-it function in bash, but it cannot be
used in dash.

Refer to other scripts that were converted together, simply change the
shebang to bash to fix the following issues when the default /bin/sh
points to other shells.

not ok 51 selftests: net: unicast_extensions.sh # exit=1

v1 -> v2:
- Fix pmtu.sh which has the same issue as unicast_extensions.sh,
suggested by Hangbin
- Change the style of the "source" line to be consistent with other
tests, suggested by Hangbin

Link: https://lore.kernel.org/all/20231202020110.362433-1-liuhangbin@gmail.com/ [1]
Link: https://lore.kernel.org/all/20231219094856.1740079-1-liuhangbin@gmail.com/ [2]
Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: 378f082eaf37 ("selftests/net: convert pmtu.sh to run it in unique namespace")
Fixes: 0f4765d0b48d ("selftests/net: convert unicast_extensions.sh to run it in unique namespace")
Signed-off-by: Yujie Liu <yujie.liu@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20231229131931.3961150-1-yujie.liu@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests/net: convert unicast_extensions.sh to run it in unique namespace

Here is the test result after conversion.

# ./unicast_extensions.sh
/usr/bin/which: no nettest in (/root/.local/bin:/root/bin:/usr/share/Modules/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin)
###########################################################################
Unicast address extensions tests (behavior of reserved IPv4 addresses)
###########################################################################
TEST: assign and ping within 240/4 (1 of 2) (is allowed) [ OK ]
TEST: assign and ping within 240/4 (2 of 2) (is allowed) [ OK ]
TEST: assign and ping within 0/8 (1 of 2) (is allowed) [ OK ]

...

TEST: assign and ping class D address (is forbidden) [ OK ]
TEST: routing using class D (is forbidden) [ OK ]
TEST: routing using 127/8 (is forbidden) [ OK ]

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

selftests/net: Use kselftest skip code for skipped tests

There are several test cases in the net directory are still using
exit 0 or exit 1 when they need to be skipped. Use kselftest
framework skip code instead so it can help us to distinguish the
return status.

Criterion to filter out what should be fixed in net directory:
grep -r "exit [01]" -B1 | grep -i skip

This change might cause some false-positives if people are running
these test scripts directly and only checking their return codes,
which will change from 0 to 4. However I think the impact should be
small as most of our scripts here are already using this skip code.
And there will be no such issue if running them with the kselftest
framework.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20210823085854.40216-1-po-hsu.lin@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: Lowest IPv4 address in a subnet is valid

Expect the lowest IPv4 address in a subnet to be assignable
and addressable as a unicast (non-broadcast) address on a
local network segment.

Signed-off-by: Seth David Schoen <schoen@loyalty.org>
Suggested-by: John Gilmore <gnu@toad.com>
Acked-by: Dave Taht <dave.taht@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

selftests: add IPv4 unicast extensions tests

Add selftests for kernel behavior with regard to various classes of
unallocated/reserved IPv4 addresses, checking whether or not these
addresses can be assigned as unicast addresses on links and used in
routing.

Expect the current kernel behavior at the time of this patch. That is:

* 0/8 and 240/4 may be used as unicast, with the exceptions of 0.0.0.0
and 255.255.255.255;
* the lowest address in a subnet may only be used as a broadcast address;
* 127/8 may not be used as unicast (the route_localnet option, which is
disabled by default, still leaves it treated slightly specially);
* 224/4 may not be used as unicast.

Signed-off-by: Seth David Schoen <schoen@loyalty.org>
Suggested-by: John Gilmore <gnu@toad.com>
Acked-by: Dave Taht <dave.taht@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20210126040834.GR24989@frotz.zork.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>