selftests: mptcp: join: skip if MPTCP is not supported

Selftests are supposed to run on any kernels, including the old ones not
supporting MPTCP.

A new check is then added to make sure MPTCP is supported. If not, the
test stops and is marked as "skipped".

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: b08fbf24 ("selftests: add test-cases for MPTCP MP_JOIN")
Cc: stable@vger.kernel.org
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -10,6 +10,8 @@
 # because it's invoked by variable name, see how the "tests" array is used
 #shellcheck disable=SC2317
 
+. "$(dirname "${0}")/mptcp_lib.sh"
+
 ret=0
 sin=""
 sinfail=""
@@ -137,6 +139,8 @@ cleanup_partial()
 
 check_tools()
 {
+	mptcp_lib_check_mptcp
+
 	if ! ip -Version &> /dev/null; then
 		echo "SKIP: Could not run test without ip tool"
 		exit $ksft_skip