zram: fix unused 'zram_wb_devops' warning

drivers/block/zram/zram_drv.c:55:45: warning: 'zram_wb_devops' defined but not used [-Wunused-const-variable=]

Fix the above warning if CONFIG_ZRAM_WRITEBACK not enabled.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kefeng Wang <[e-mail]>
Reviewed-by: Sergey Senozhatsky <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Nitin Gupta <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/block/zram/zram_drv.c

···
 static size_t huge_class_size;
 
 static const struct block_device_operations zram_devops;
+#ifdef CONFIG_ZRAM_WRITEBACK
 static const struct block_device_operations zram_wb_devops;
+#endif
 
 static void zram_free_page(struct zram *zram, size_t index);
 static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,