dm: fix comment in dm_process_bio()

Refer to the correct function (->submit_bio instead of ->queue_bio).
Also, add details about why using blk_queue_split() isn't needed for
dm_wq_work()'s call to dm_process_bio().

Fixes: c62b37d9 ("block: move ->make_request_fn to struct block_device_operations")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1744,9 +1744,11 @@ static blk_qc_t dm_process_bio(struct mapped_device *md,
 	}
 
 	/*
-	 * If in ->queue_bio we need to use blk_queue_split(), otherwise
+	 * If in ->submit_bio we need to use blk_queue_split(), otherwise
 	 * queue_limits for abnormal requests (e.g. discard, writesame, etc)
 	 * won't be imposed.
+	 * If called from dm_wq_work() for deferred bio processing, bio
+	 * was already handled by following code with previous ->submit_bio.
 	 */
 	if (current->bio_list) {
 		if (is_abnormal_io(bio))