blk-mq: Add locking annotations to hctx_lock() and hctx_unlock()

This patch avoids that sparse reports the following:

block/blk-mq.c:637:33: warning: context imbalance in 'hctx_unlock' - unexpected unlock
block/blk-mq.c:642:9: warning: context imbalance in 'hctx_lock' - wrong count at exit
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -559,6 +559,7 @@ static void __blk_mq_complete_request(struct request *rq)
 }
 
 static void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx)
+	__releases(hctx->srcu)
 {
 	if (!(hctx->flags & BLK_MQ_F_BLOCKING))
 		rcu_read_unlock();
@@ -567,6 +568,7 @@ static void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx)
 }
 
 static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx)
+	__acquires(hctx->srcu)
 {
 	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
 		/* shut up gcc false positive */