blk-mq: short cut the IPI path in blk_mq_force_complete_rq for !SMP

Let the compile optimize out the entire IPI path, given that we are
obviously not going to use it.
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -727,7 +727,8 @@ void blk_mq_force_complete_rq(struct request *rq)
 		return;
 	}
 
-	if (!test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags)) {
+	if (!IS_ENABLED(CONFIG_SMP) ||
+	    !test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags)) {
 		q->mq_ops->complete(rq);
 		return;
 	}