specfs: Prevent new opens while close is waiting to drain.

Otherwise, bdev/cdev_close could have cancelled all _existing_ opens,
and waited for them to complete (and freed resources used by them) --
but a new one could start, and hang (e.g., a tty), at the same time
spec_close tries to drain all pending I/O operations, one of which
(the new open) is now hanging indefinitely.

Preventing the new open from even starting until bdev/cdev_close is
finished and all I/O operations have drained avoids this deadlock.

(riastradh)