selftests: mptcp_connect: add SO_TIMESTAMPNS cmsg support

This extends the existing setsockopt test case to also check for cmsg
timestamps.

mptcp_connect will abort/fail if the setockopt was passed but the
timestamp cmsg isn't present after successful recvmsg().

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>