selftests/net: Improve bind_bhash.sh to accommodate predictable network interface names

Starting with v197, systemd uses predictable interface network names,
the traditional interface naming scheme (eth0) is deprecated, therefore
it cannot be assumed that the eth0 interface exists on the host.

This modification makes the bind_bhash test program run in a separate
network namespace and no longer needs to consider the name of the
network interface on the host.

Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

selftests/net: Add test for timing a bind request to a port with a populated bhash entry

This test populates the bhash table for a given port with
MAX_THREADS * MAX_CONNECTIONS sockets, and then times how long
a bind request on the port takes.

When populating the bhash table, we create the sockets and then bind
the sockets to the same address and port (SO_REUSEADDR and SO_REUSEPORT
are set). When timing how long a bind on the port takes, we bind on a
different address without SO_REUSEPORT set. We do not set SO_REUSEPORT
because we are interested in the case where the bind request does not
go through the tb->fastreuseport path, which is fragile (eg
tb->fastreuseport path does not work if binding with a different uid).

To run the script:
Usage: ./bind_bhash.sh [-6 | -4] [-p port] [-a address]
6: use ipv6
4: use ipv4
port: Port number
address: ip address

Without any arguments, ./bind_bhash.sh defaults to ipv6 using ip address
"2001:0db8:0:f101::1" on port 443.

On my local machine, I see:
ipv4:
before - 0.002317 seconds
with bhash2 - 0.000020 seconds

ipv6:
before - 0.002431 seconds
with bhash2 - 0.000021 seconds

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>