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/[e-mail]
Signed-off-by: Yue Hu <[e-mail]>
Reviewed-by: Sergey Senozhatsky <[e-mail]>
Acked-by: Minchan Kim <[e-mail]>
Cc: Sergey Senozhatsky <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/zram/zram_drv.h

···
 	 * 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;