selftests: mptcp: sockopt: skip getsockopt checks if not supported

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

One of them is the getsockopt(SOL_MPTCP) to get info about the MPTCP
connections introduced by commit 55c42fa7fa ("mptcp: add MPTCP_INFO
getsockopt") and the following ones.

It is possible to look for "mptcp_diag_fill_info" in kallsyms because
it is introduced by the mentioned feature. So we can know in advance if
the feature is supported and skip the sub-test if not.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: ce9979129a ("selftests: mptcp: add mptcp getsockopt test cases")
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/mptcp_sockopt.sh

@@ -87,6 +87,7 @@ cleanup()
 }
 
 mptcp_lib_check_mptcp
+mptcp_lib_check_kallsyms
 
 ip -Version > /dev/null 2>&1
 if [ $? -ne 0 ];then
@@ -253,6 +254,11 @@ do_mptcp_sockopt_tests()
 {
 	local lret=0
 
+	if ! mptcp_lib_kallsyms_has "mptcp_diag_fill_info$"; then
+		echo "INFO: MPTCP sockopt not supported: SKIP"
+		return
+	fi
+
 	ip netns exec "$ns_sbox" ./mptcp_sockopt
 	lret=$?