selftests: mptcp: pm nl: avoid error msg on older kernels

Since the 'Fixes' commit mentioned below, and if the kernel being tested
doesn't support the 'fullmesh' flag, this error will be printed:

  netlink error -22 (Invalid argument)
  ./pm_nl_ctl: bailing out due to netlink error[s]

But that can be normal if the kernel doesn't support the feature, no
need to print this worrying error message while everything else looks
OK. So we can mute stderr. Failures will still be detected if any.

Fixes: 1dc88d24 ("selftests: mptcp: pm_nl_ctl: always look for errors")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -183,7 +183,7 @@ check "ip netns exec $ns1 ./pm_nl_ctl dump" "id 1 flags \
 subflow 10.0.1.1" "          (nobackup)"
 
 # fullmesh support has been added later
-ip netns exec $ns1 ./pm_nl_ctl set id 1 flags fullmesh
+ip netns exec $ns1 ./pm_nl_ctl set id 1 flags fullmesh 2>/dev/null
 if ip netns exec $ns1 ./pm_nl_ctl dump | grep -q "fullmesh" ||
    mptcp_lib_expect_all_features; then
 	check "ip netns exec $ns1 ./pm_nl_ctl dump" "id 1 flags \