blk-mq: fix sparse warning on missed __percpu annotation

'struct blk_mq_ctx' is  __percpu, so add the annotation
and fix the sparse warning reported from Fengguang:

	[block:for-linus 2/3] block/blk-mq.h:75:16: sparse: incorrect
	type in initializer (different address spaces)
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -1767,7 +1767,7 @@ static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 {
 	struct blk_mq_hw_ctx **hctxs;
-	struct blk_mq_ctx *ctx;
+	struct blk_mq_ctx __percpu *ctx;
 	struct request_queue *q;
 	unsigned int *map;
 	int i;

include/linux/blkdev.h

@@ -335,7 +335,7 @@ struct request_queue {
 	unsigned int		*mq_map;
 
 	/* sw queues */
-	struct blk_mq_ctx	*queue_ctx;
+	struct blk_mq_ctx __percpu	*queue_ctx;
 	unsigned int		nr_queues;
 
 	/* hw dispatch queues */