blk-mq: update hctx->dispatch_busy in case of real scheduler

Commit 6e6fcbc2 ("blk-mq: support batching dispatch in case of io")
starts to support io batching submission by using hctx->dispatch_busy.

However, blk_mq_update_dispatch_busy() isn't changed to update hctx->dispatch_busy
in that commit, so fix the issue by updating hctx->dispatch_busy in case
of real scheduler.
Reported-by: NJan Kara <jack@suse.cz>
Reviewed-by: NJan Kara <jack@suse.cz>
Fixes: 6e6fcbc2 ("blk-mq: support batching dispatch in case of io")
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210625020248.1630497-1-ming.lei@redhat.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -1224,9 +1224,6 @@ static void blk_mq_update_dispatch_busy(struct blk_mq_hw_ctx *hctx, bool busy)
 {
 	unsigned int ewma;
 
-	if (hctx->queue->elevator)
-		return;
-
 	ewma = hctx->dispatch_busy;
 
 	if (!ewma && !busy)