selftests: drv-net: wait for iperf client to stop sending

A few packets may still be sent out during the termination of iperf
processes. These late packets cause failures in rss_ctx.py when they
arrive on queues expected to be empty.

Example failure observed:

  Check failed 2 != 0 traffic on inactive queues (context 1):
    [0, 0, 1, 1, 386385, 397196, 0, 0, 0, 0, ...]

  Check failed 4 != 0 traffic on inactive queues (context 2):
    [0, 0, 0, 0, 2, 2, 247152, 253013, 0, 0, ...]

  Check failed 2 != 0 traffic on inactive queues (context 3):
    [0, 0, 0, 0, 0, 0, 1, 1, 282434, 283070, ...]

To avoid such failures, wait until all client sockets for the requested
port are either closed or in the TIME_WAIT state.

Fixes: 847aa551 ("selftests: drv-net: rss_ctx: factor out send traffic and check")
Signed-off-by: Nimrod Oren <noren@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250722122655.3194442-1-noren@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>