selftests: mptcp: diag: 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: df62f2ec ("selftests/mptcp: add diag interface tests")
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/diag.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+. "$(dirname "${0}")/mptcp_lib.sh"
+
 sec=$(date +%s)
 rndh=$(printf %x $sec)-$(mktemp -u XXXXXX)
 ns="ns1-$rndh"
@@ -31,6 +33,8 @@ cleanup()
 	ip netns del $ns
 }
 
+mptcp_lib_check_mptcp
+
 ip -Version > /dev/null 2>&1
 if [ $? -ne 0 ];then
 	echo "SKIP: Could not run test without ip tool"