block: add blk_queue_dead()

There are a number of QUEUE_FLAG_DEAD tests.  Add blk_queue_dead()
macro and use it.

This patch doesn't introduce any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-exec.c b/block/blk-exec.c
index a1ebceb..6053285 100644
--- a/block/blk-exec.c
+++ b/block/blk-exec.c
@@ -50,7 +50,7 @@
 {
 	int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
 
-	if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
+	if (unlikely(blk_queue_dead(q))) {
 		rq->errors = -ENXIO;
 		if (rq->end_io)
 			rq->end_io(rq, rq->errors);