selftests/net: udpgso_bench_rx: fix port argument

[ Upstream commit d336509c ]

The below commit added optional support for passing a bind address.
It configures the sockaddr bind arguments before parsing options and
reconfigures on options -b and -4.

This broke support for passing port (-p) on its own.

Configure sockaddr after parsing all arguments.

Fixes: 3327a9c4

 ("selftests: add functionals test for UDP GRO")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>