Replace global queue and lock with per-device queues and locks.  Mostly a
straightforward replacement of ubd_io_lock with dev->lock and ubd_queue with
dev->queue.

Complications -
	There was no way to get a request struct (and queue) from the
structure sent to the io_thread, so a pointer to the request was
added.  This is needed in ubd_handler in order to kick do_ubd_request
to process another request.
	Queue initialization is moved from ubd_init to ubd_add.
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>