ANDROID: zram: Refactor writeback helpers

Move and export writeback-related definitions and functions from
zram_drv.c to zram_drv.h.

This change is a pure refactoring and introduces no functional
differences. It prepares the codebase for an upcoming ioctl that
will need to access these helpers.

Bug: 421065577
Change-Id: Ibc7d3825c76294ecf30c25088a2c02b0b2447da4
Signed-off-by: Richard Chang <richardycc@google.com>