Fixed failure of clearing loopback device

Loopback device is already cleared when resetting ZRAM_BACK_DEV,
ZRAM_BACK_DEV is changed to "none" from "/dev/block/loopXX".
So it needs to read ZRAM_BACK_DEV again before checking it.

Bug: 388158025
Test:
- Boot up device with zram writeback enabled
- adb root
- adb reboot userspace

Change-Id: Ib7b648f6a50eb3ef4d3b0268ed9e11d8c6e0a181