selftests: mptcp: userspace pm: skip if 'ip' tool is unavailable

When a required tool is missing, the return code 4 (SKIP) should be
returned instead of 1 (FAIL).

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 259a834fadda ("selftests: mptcp: functional tests for the userspace PM type")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -8,7 +8,7 @@ mptcp_lib_check_mptcp
 ip -Version > /dev/null 2>&1
 if [ $? -ne 0 ];then
 	echo "SKIP: Cannot not run test without ip tool"
-	exit 1
+	exit ${KSFT_SKIP}
 fi
 
 ANNOUNCED=6        # MPTCP_EVENT_ANNOUNCED