nbd: fix sparse warning

I just fixed this in "drivers/block/rbd.c" and I noticed that
"drivers/block/nbd.c" has the same problem. Fix a warning issued by
sparse by adding some lockdep annotations to indicate the queue lock gets
dropped (because it's held when do_nbd_request() is called) and
re-acquired within the function.

Signed-off-by: Alex Elder <[e-mail]>
Cc: Paul Clements <[e-mail]>
Cc: Paul Clements <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/nbd.c

···
  */
 
 static void do_nbd_request(struct request_queue *q)
+		__releases(q->queue_lock) __acquires(q->queue_lock)
 {
 	struct request *req;