mptcp: avoid subflow socket usage in mptcp_get_port()

We are going to remove the first subflow socket soon, so avoid
accessing it in mptcp_get_port(). Instead, access directly the
first subflow sock.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>