When deleting netns, it is possible to still have some tasks running,
e.g. background tasks like tcpdump running in the background, not
stopped because the test has been interrupted.

Before deleting the netns, it is then safer to kill all attached PIDs,
if any. That should reduce some noises after the end of some tests, and
help with the debugging of some issues. That's why this modification is
seen as a "fix".

Fixes: 25ae948b ("selftests/net: add lib.sh")
Acked-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Acked-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20240813-upstream-net-20240813-selftests-net-lib-kill-v1-1-27b689b248b8@kernel.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>