selftests, bpf: Fix test_tc_tunnel hanging

When run_kselftests.sh is run, it hangs after test_tc_tunnel.sh. The reason
is test_tc_tunnel.sh ensures the server ('nc -l') is run all the time,
starting it again every time it is expected to terminate. The exception is
the final client_connect: the server is not started anymore, which ensures
no process is kept running after the test is finished.

For a sit test, though, the script is terminated prematurely without the
final client_connect and the 'nc' process keeps running. This in turn causes
the run_one function in kselftest/runner.sh to hang forever, waiting for the
runaway process to finish.

Ensure a remaining server is terminated on cleanup.

Fixes: f6ad6accaa ("selftests/bpf: expand test_tc_tunnel with SIT encap")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/bpf/60919291657a9ee89c708d8aababc28ebe1420be.1573821780.git.jbenc@redhat.com

tools/testing/selftests/bpf/test_tc_tunnel.sh

@@ -62,6 +62,10 @@ cleanup() {
 	if [[ -f "${infile}" ]]; then
 		rm "${infile}"
 	fi
+
+	if [[ -n $server_pid ]]; then
+		kill $server_pid 2> /dev/null
+	fi
 }
 
 server_listen() {
@@ -77,6 +81,7 @@ client_connect() {
 
 verify_data() {
 	wait "${server_pid}"
+	server_pid=
 	# sha1sum returns two fields [sha1] [filepath]
 	# convert to bash array and access first elem
 	insum=($(sha1sum ${infile}))