selftests: net: more pmtu.sh fixes

The netdev CI is reporting failures for the pmtu test:

  [  115.929264] br0: port 2(vxlan_a) entered forwarding state
  # 2024/02/08 17:33:22 socat[7871] E bind(7, {AF=10 [0000:0000:0000:0000:0000:0000:0000:0000]:50000}, 28): Address already in use
  # 2024/02/08 17:33:22 socat[7877] E write(7, 0x5598fb6ff000, 8192): Connection refused
  # TEST: IPv6, bridged vxlan4: PMTU exceptions                         [FAIL]
  # File size 0 mismatches exepcted value in locally bridged vxlan test

The root cause is apparently a socket created by a previous iteration
of the relevant loop still lasting in LAST_ACK state.

Note that even the file size check is racy, the receiver process dumping
the file could still be running in background

Allow the listener to bound on the same local port via SO_REUSEADDR and
collect file output file size only after the listener completion.

Fixes: 136a1b43

 ("selftests: net: test vxlan pmtu exceptions with tcp")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/4f51c11a1ce7ca7a4dabd926cffff63dadac9ba1.1707731086.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>