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-core.c b/block/blk-core.c
index 435af23..b5ed4f4 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -608,7 +608,7 @@
 
 int blk_get_queue(struct request_queue *q)
 {
-	if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
+	if (likely(!blk_queue_dead(q))) {
 		kobject_get(&q->kobj);
 		return 0;
 	}
@@ -755,7 +755,7 @@
 	const bool is_sync = rw_is_sync(rw_flags) != 0;
 	int may_queue;
 
-	if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
+	if (unlikely(blk_queue_dead(q)))
 		return NULL;
 
 	may_queue = elv_may_queue(q, rw_flags);
@@ -875,7 +875,7 @@
 		struct io_context *ioc;
 		struct request_list *rl = &q->rq;
 
-		if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
+		if (unlikely(blk_queue_dead(q)))
 			return NULL;
 
 		prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,