netdevsim: Fix hwstats debugfs file permissions

The hwstats debugfs files are only writeable, but they are created with
read and write permissions, causing certain selftests to fail [1].

Fix by creating the files with write permission only.

[1]
# ./test_offload.py
Test destruction of generic XDP...
Traceback (most recent call last):
File "/home/idosch/code/linux/tools/testing/selftests/bpf/./test_offload.py", line 810, in <module>
simdev = NetdevSimDev()
[...]
Exception: Command failed: cat /sys/kernel/debug/netdevsim/netdevsim0//ports/0/dev/hwstats/l3/disable_ifindex

cat: /sys/kernel/debug/netdevsim/netdevsim0//ports/0/dev/hwstats/l3/disable_ifindex: Invalid argument

Fixes: 1a6d7ae7d63c ("netdevsim: Introduce support for L3 offload xstats")
Reported-by: Jie2x Zhou <jie2x.zhou@intel.com>
Tested-by: Jie2x Zhou <jie2x.zhou@intel.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20220909153830.3732504-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

netdevsim: Introduce support for L3 offload xstats

Add support for testing of HW stats support that was added recently, namely
the L3 stats support. L3 stats are provided for devices for which the L3
stats have been turned on, and that were enabled for netdevsim through a
debugfs toggle:

# echo $ifindex > /sys/kernel/debug/netdevsim/$DEV/hwstats/l3/enable_ifindex

For fully enabled netdevices, netdevsim counts 10pps of ingress traffic and
20pps of egress traffic. Similarly, L3 stats can be disabled for a given
device, and netdevsim ceases pretending there is any HW traffic going on:

# echo $ifindex > /sys/kernel/debug/netdevsim/$DEV/hwstats/l3/disable_ifindex

Besides this, there is a third toggle to mark a device for future failure:

# echo $ifindex > /sys/kernel/debug/netdevsim/$DEV/hwstats/l3/fail_next_enable

A future request to enable L3 stats on such netdevice will be bounced by
netdevsim:

# ip -j l sh dev d | jq '.[].ifindex'
66
# echo 66 > /sys/kernel/debug/netdevsim/netdevsim10/hwstats/l3/enable_ifindex
# echo 66 > /sys/kernel/debug/netdevsim/netdevsim10/hwstats/l3/fail_next_enable
# ip stats set dev d l3_stats on
Error: netdevsim: Stats enablement set to fail.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>