drivers/block/null_blk_main.c: fix uninitialized var warnings

With gcc-7.2, many instances of

drivers/block/null_blk_main.c: In function ‘nullb_device_zone_nr_conv_store’:
drivers/block/null_blk_main.c:291:12: warning: ‘new_value’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  dev->NAME = new_value;      \
            ^
drivers/block/null_blk_main.c:279:7: note: ‘new_value’ was declared here
  TYPE new_value;       \
       ^

Presumably notabug, so use uninitialized_var() to suppress them.

Cc: Shaohua Li <shli@fb.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/null_blk_main.c

@@ -276,7 +276,7 @@ nullb_device_##NAME##_store(struct config_item *item, const char *page,	\
 {									\
 	int (*apply_fn)(struct nullb_device *dev, TYPE new_value) = APPLY;\
 	struct nullb_device *dev = to_nullb_device(item);		\
-	TYPE new_value;							\
+	TYPE uninitialized_var(new_value);				\
 	int ret;							\
 									\
 	ret = nullb_device_##TYPE##_attr_store(&new_value, page, count);\