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>