mptcp: avoid ssock usage in mptcp_pm_nl_create_listen_socket()

This is one of the few remaining spots actually manipulating the
first subflow socket. We can leverage the recently introduced
inet helpers to get rid of ssock there.

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>