block: only mark bio as tracked if it really is tracked

We set BIO_TRACKED unconditionally when rq_qos_throttle() is called, even
though we may not even have an rq_qos handler. Only mark it as TRACKED if
it really is potentially tracked.

This saves considerable time for the case where the bio isn't tracked:

     2.64%     -1.65%  [kernel.vmlinux]  [k] bio_endio
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-rq-qos.h

@@ -189,9 +189,10 @@ static inline void rq_qos_throttle(struct request_queue *q, struct bio *bio)
 	 * BIO_TRACKED lets controllers know that a bio went through the
 	 * normal rq_qos path.
 	 */
-	bio_set_flag(bio, BIO_TRACKED);
-	if (q->rq_qos)
+	if (q->rq_qos) {
+		bio_set_flag(bio, BIO_TRACKED);
 		__rq_qos_throttle(q->rq_qos, bio);
+	}
 }
 
 static inline void rq_qos_track(struct request_queue *q, struct request *rq,