dm rq: Avoid that request processing stalls sporadically

While running the srp-test software I noticed that request
processing stalls sporadically at the beginning of a test, namely
when mkfs is run against a dm-mpath device. Every time when that
happened the following command was sufficient to resume request
processing:

echo run >/sys/kernel/debug/block/dm-0/state

This patch avoids that such request processing stalls occur. The
test I ran is as follows:

while srp-test/run_tests -d -r 30 -t 02-mq; do :; done

Signed-off-by: Bart Van Assche <[e-mail]>
Cc: Mike Snitzer <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/md/dm-rq.c

···
 		/* Undo dm_start_request() before requeuing */
 		rq_end_stats(md, rq);
 		rq_completed(md, rq_data_dir(rq), false);
+		blk_mq_delay_run_hw_queue(hctx, 100/*ms*/);
 		return BLK_MQ_RQ_QUEUE_BUSY;
 	}