mptcp: fastclose msk when cleaning unaccepted sockets

When cleaning up unaccepted mptcp socket still laying inside
the listener queue at listener close time, such sockets will
go through a regular close, waiting for a timeout before
shutting down the subflows.

There is no need to keep the kernel resources in use for
such a possibly long time: short-circuit to fast-close.

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