dm-rq: don't dereference request payload after ending request

Bart reported a case where dm would crash with use-after-free
poison. This is due to dm_softirq_done() accessing memory
associated with a request after calling end_request on it.
This is most visible on !blk-mq, since we free the memory
immediately for that case.
Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Fixes: eb8db831 ("dm: always defer request allocation to the owner of the request_queue")
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/md/dm-rq.c

@@ -328,13 +328,15 @@ static void dm_softirq_done(struct request *rq)
 	int rw;
 
 	if (!clone) {
-		rq_end_stats(tio->md, rq);
+		struct mapped_device *md = tio->md;
+
+		rq_end_stats(md, rq);
 		rw = rq_data_dir(rq);
 		if (!rq->q->mq_ops)
 			blk_end_request_all(rq, tio->error);
 		else
 			blk_mq_end_request(rq, tio->error);
-		rq_completed(tio->md, rw, false);
+		rq_completed(md, rw, false);
 		return;
 	}