null_blk: add lock drop/acquire annotation

sparse complains:

drivers/block/null_blk_main.c:816:24: sparse: context imbalance in 'null_insert_page' - unexpected unlock

Fix it by adding the necessary annotations to the function.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -805,6 +805,8 @@ static struct nullb_page *null_lookup_page(struct nullb *nullb,
 
 static struct nullb_page *null_insert_page(struct nullb *nullb,
 					   sector_t sector, bool ignore_cache)
+	__releases(&nullb->lock)
+	__acquires(&nullb->lock)
 {
 	u64 idx;
 	struct nullb_page *t_page;