blk-mq: provide mq_ops->busy() hook

We'll hook into this from blk_lld_busy(), allowing blk-mq to also
return whether or not a given queue currently has requests in
progress.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-core.c b/block/blk-core.c
index ce12515f..ca1a3af 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -3431,6 +3431,8 @@ int blk_lld_busy(struct request_queue *q)
 {
 	if (q->lld_busy_fn)
 		return q->lld_busy_fn(q);
+	if (q->mq_ops && q->mq_ops->busy)
+		return q->mq_ops->busy(q);
 
 	return 0;
 }