blk-mq: optimise *end_request non-stat path

We already have a blk_mq_need_time_stamp() check in
__blk_mq_end_request() to get a timestamp, hide all the statistics
accounting under it. It cuts some cycles for requests that don't need
stats, and is free otherwise.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/e0f2ea812e93a8adcd07101212e7d7e70ca304e7.1634115360.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -579,10 +579,8 @@ void blk_mq_free_plug_rqs(struct blk_plug *plug)
 
 inline void __blk_mq_end_request(struct request *rq, blk_status_t error)
 {
-	u64 now = 0;
-
-	if (blk_mq_need_time_stamp(rq))
-		now = ktime_get_ns();
+	if (blk_mq_need_time_stamp(rq)) {
+		u64 now = ktime_get_ns();
 
 		if (rq->rq_flags & RQF_STATS) {
 			blk_mq_poll_stats_start(rq->q);
@@ -590,8 +588,8 @@ inline void __blk_mq_end_request(struct request *rq, blk_status_t error)
 		}
 
 		blk_mq_sched_completed_request(rq, now);
-
 		blk_account_io_done(rq, now);
+	}
 
 	if (rq->end_io) {
 		rq_qos_done(rq->q, rq);