ANDROID: zram: Add ioctl for per-process writeback operation

Introduce a new ioctl command `ZRAM_ANDROID_IOC_PROCESS_WRITEBACK`.
Operate on a target process identified by a pidfd, and then iterates
through the virtual memory areas (VMAs) of the target process,
identifies pages that are swapped to the zram device, and then invokes
the corresponding writeback operation.

Bug: 421065577
Change-Id: I84cedb0c18a8088114582e7f54637e119fad1564
Signed-off-by: Richard Chang <richardycc@google.com>
(cherry picked from commit d174eba7)
Bug: 446676432
Signed-off-by: David Stevens <stevensd@google.com>