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>