mm: process_reclaim: support reclaim of driver owned pages

GPU drivers can allocate memory via shmem and these pages
may or may not be mapped to userspace at any moment. But
these pages are allocated on behalf of userspace tasks.
It is ideal to swapout the unmapped pages of a task during
process reclaim. But as process reclaims works on mappings
to derive pages, it misses the unmapped gpu pages. Add an
option to reclaim these pages. Drivers can register to the
reclaim notifier and call into the reclaim function with the
mapping that contains these mapped/unmapped pages.

Change-Id: Ia5de99d9830c0bdf0e9e56d860987f79f95df435
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>