KillZramBackingDevice: Return immediately if backing_dev is none.

It's possible that CONFIG_ZRAM_WRITEBACK is y,
but userspace doesn't set the /sys/block/zram0/backing_dev,
so its value is 'none'.
It's the same with "CONFIG_ZRAM_WRITEBACK is not set".

Change-Id: I2df89ceee68e4685deef5113bada21be96779e9b
Signed-off-by: shisiyuan <shisiyuan@xiaomi.com>