blk-mq: make lifetime consistent between hctx and its kobject

This patch removes kobject_put() over hctx in __blk_mq_unregister_dev(),
and trys to keep lifetime consistent between hctx and hctx's kobject.

Now blk_mq_sysfs_register() and blk_mq_sysfs_unregister() become
totally symmetrical, and kobject's refcounter drops to zero just
when the hctx is freed.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index b985c23..f70595e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2256,8 +2256,7 @@
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (!hctx)
 			continue;
-		kfree(hctx->ctxs);
-		kfree(hctx);
+		kobject_put(&hctx->kobj);
 	}
 
 	q->mq_map = NULL;
@@ -2336,8 +2335,6 @@
 			blk_mq_exit_hctx(q, set, hctx, j);
 			free_cpumask_var(hctx->cpumask);
 			kobject_put(&hctx->kobj);
-			kfree(hctx->ctxs);
-			kfree(hctx);
 			hctxs[j] = NULL;
 
 		}