zram: move backing_dev under macro CONFIG_ZRAM_WRITEBACK

backing_dev is never used when not enable CONFIG_ZRAM_WRITEBACK and it's
introduced from writeback feature.  So it's needless also affect
readability in that case.

Link: https://lkml.kernel.org/r/20210521060544.2385-1-zbestahu@gmail.com
Signed-off-by: Yue Hu <huyue2@yulong.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.h

@@ -113,8 +113,8 @@ struct zram {
 	 * zram is claimed so open request will be failed
 	 */
 	bool claim; /* Protected by bdev->bd_mutex */
-	struct file *backing_dev;
 #ifdef CONFIG_ZRAM_WRITEBACK
+	struct file *backing_dev;
 	spinlock_t wb_limit_lock;
 	bool wb_limit_enable;
 	u64 bd_wb_limit;