mptcp: mptcp: avoid additional indirection in mptcp_bind()

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