ANDROID: export symbols for zram module

The zram module is being updated to allow initiating writeback on behalf
of a specific userspace process. This requires the zram module to call
several core kernel functions to access the process's memory map and
walk its page tables.

Export the following symbols so they can be used by the zram module:
 - mm_access: To get a reference to the process's mm_struct.
 - pidfd_get_task: To get the task_struct from a pidfd.
 - walk_page_range: To iterate over the process's page table entries.
 - __pte_offset_map_lock: To map and lock a PTE.

This change is a prerequisite for adding the new per-process writeback
ioctl to zram.

Bug: 421065577
Change-Id: I16eb8aec0f4a8c4a23f73fba74b309cc5b4d69dd
Signed-off-by: Richard Chang <richardycc@google.com>