block: blk_queue_enter() / __bio_queue_enter() must return -EAGAIN for nowait

Today blk_queue_enter() and __bio_queue_enter() return -EBUSY for the
nowait code path. This is not correct: they should return -EAGAIN
instead.

This problem was detected by fio. The following command exposed the
above problem:

t/io_uring -p0 -d128 -b4096 -s32 -c32 -F1 -B0 -R0 -X1 -n24 -P1 -u1 -O0 /dev/ng0n1

By applying the patch, the retry case is handled correctly in the slow
path.

Signed-off-by: Stefan Roesch <shr@fb.com>
Fixes: bfd343aa17 ("blk-mq: don't wait in blk_mq_queue_enter() if __GFP_WAIT isn't set")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -295,7 +295,7 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 
 	while (!blk_try_enter_queue(q, pm)) {
 		if (flags & BLK_MQ_REQ_NOWAIT)
-			return -EBUSY;
+			return -EAGAIN;
 
 		/*
 		 * read pair of barrier in blk_freeze_queue_start(), we need to
@@ -325,7 +325,7 @@ int __bio_queue_enter(struct request_queue *q, struct bio *bio)
 			if (test_bit(GD_DEAD, &disk->state))
 				goto dead;
 			bio_wouldblock_error(bio);
-			return -EBUSY;
+			return -EAGAIN;
 		}
 
 		/*