block, bfq: make sure queue budgets are not below service received

With some unlucky sequences of events, the function bfq_updated_next_req
updates the current budget of a bfq_queue to a lower value than the
service received by the queue using such a budget. Unfortunately, if
this happens, then the return value of the function bfq_bfqq_budget_left
becomes inconsistent. This commit solves this problem by lower-bounding
the budget computed in bfq_updated_next_req to the service currently
charged to the queue.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -907,8 +907,10 @@ static void bfq_updated_next_req(struct bfq_data *bfqd,
 		 */
 		return;
 
-	new_budget = max_t(unsigned long, bfqq->max_budget,
-			   bfq_serv_to_charge(next_rq, bfqq));
+	new_budget = max_t(unsigned long,
+			   max_t(unsigned long, bfqq->max_budget,
+				 bfq_serv_to_charge(next_rq, bfqq)),
+			   entity->service);
 	if (entity->budget != new_budget) {
 		entity->budget = new_budget;
 		bfq_log_bfqq(bfqd, bfqq, "updated next rq: new budget %lu",