mptcp: avoid additional indirection in mptcp_listen()

We are going to remove the first subflow socket soon, so avoid
the additional indirection via at listen() time. Instead call
directly the recently introduced helper on the first subflow sock.

No functional changes intended.

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>