ANDROID: zram: fix incorrect assignment for access time

The backports for zram writeback feature did not account for
changes in ktime_t definition. ktime_t is a union on 4.9.

Test: Build with allmodconfig
Change-Id: I56d521e25727398fc02e658f033bcb141e610d34
Signed-off-by: Srinivas Paladugu <srnvs@google.com>