ANDROID: vendor_hook: Add hook in shmem_writepage()

Add vendor hook android_vh_set_shmem_page_flag in shmem_writepage to
set a flag in page_ext, which indicates that this page is a shmem page,
to be used in android_vh_get_swap_page. The shared page should not be
reclaimed to the extended memory, ie, the specified swap location.

Bug: 234214858
Signed-off-by: Bing Han <bing.han@transsion.com>
Change-Id: I33a9007c88b4d8aab3da044c8a05eb45d7e74f3a