blk-mq: fix hang in bt_get()

Avoid that if there are fewer hardware queues than CPU threads that
bt_get() can hang. The symptoms of the hang were as follows:

* All tags allocated for a particular hardware queue.
* (nr_tags) pending commands for that hardware queue.
* No pending commands for the software queues associated with that
  hardware queue.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq-tag.c

@@ -254,6 +254,13 @@ static int bt_get(struct blk_mq_alloc_data *data,
 		if (tag != -1)
 			break;
 
+		/*
+		 * We're out of tags on this hardware queue, kick any
+		 * pending IO submits before going to sleep waiting for
+		 * some to complete.
+		 */
+		blk_mq_run_hw_queue(hctx, false);
+
 		blk_mq_put_ctx(data->ctx);
 
 		io_schedule();