zram: only make zram_wb_devops for CONFIG_ZRAM_WRITEBACK

If writeback isn't configured, then we get the following warning when
compiling zram:

drivers/block/zram/zram_drv.c:1824:45: warning: unused variable 'zram_wb_devops' [-Wunused-const-variable]

Make sure we only define the block_device_operations if that option is
enabled.

Link: https://lore.kernel.org/lkml/202111261614.gCJMqcyh-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/zram/zram_drv.c

@@ -1853,12 +1853,14 @@ static const struct block_device_operations zram_devops = {
 	.owner = THIS_MODULE
 };
 
+#ifdef CONFIG_ZRAM_WRITEBACK
 static const struct block_device_operations zram_wb_devops = {
 	.open = zram_open,
 	.submit_bio = zram_submit_bio,
 	.swap_slot_free_notify = zram_slot_free_notify,
 	.owner = THIS_MODULE
 };
+#endif
 
 static DEVICE_ATTR_WO(compact);
 static DEVICE_ATTR_RW(disksize);