fcnal-test: kill hanging ping/nettest binaries on cleanup

On my box I see a bunch of ping/nettest processes hanging
around after fcntal-test.sh is done.

Clean those up before netns deletion.
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20211021140247.29691-1-fw@strlen.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/fcnal-test.sh

@@ -445,10 +445,13 @@ cleanup()
 		ip -netns ${NSA} link set dev ${NSA_DEV} down
 		ip -netns ${NSA} link del dev ${NSA_DEV}
 
+		ip netns pids ${NSA} | xargs kill 2>/dev/null
 		ip netns del ${NSA}
 	fi
 
+	ip netns pids ${NSB} | xargs kill 2>/dev/null
 	ip netns del ${NSB}
+	ip netns pids ${NSC} | xargs kill 2>/dev/null
 	ip netns del ${NSC} >/dev/null 2>&1
 }