block, bfq-mq: turn BUG_ON on request-size into WARN_ON

BFQ has many checks of internal and external consistency. One of them
checks that an I/O request has still sectors to serve, if it happens
to be retired without being served. If the request has no sector to
serve, a BUG_ON signals the failure and causes the kernel to
terminate. Yet, from a crash report by a user [1], this condition may
happen to hold, in apparently correct functioning, for I/O with a
CD/DVD.

To address this issue, this commit turns the above BUG_ON into a
WARN_ON. This commit also adds a companion WARN_ON on request
insertion into the scheduler.

[1] https://groups.google.com/d/msg/bfq-iosched/DDOTJBroBa4/VyU1zUFtCgAJ

Reported-by: Alexandre Frade <admfrade@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>

block/bfq-mq-iosched.c

@@ -1540,6 +1540,8 @@ static void bfq_add_request(struct request *rq)
 
 	BUG_ON(!RQ_BFQQ(rq));
 	BUG_ON(RQ_BFQQ(rq) != bfqq);
+	WARN_ON(blk_rq_sectors(rq) == 0);
+
 	elv_rb_add(&bfqq->sort_list, rq);
 
 	/*
@@ -4962,7 +4964,7 @@ static void bfq_finish_request(struct request *rq)
 					     rq_io_start_time_ns(rq),
 					     rq->cmd_flags);
 
-	BUG_ON(blk_rq_sectors(rq) == 0 && !(rq->rq_flags & RQF_STARTED));
+	WARN_ON(blk_rq_sectors(rq) == 0 && !(rq->rq_flags & RQF_STARTED));
 
 	if (likely(rq->rq_flags & RQF_STARTED)) {
 		unsigned long flags;