mm: per-process reclaim

These day, there are many platforms available in the embedded market
and they are smarter than kernel which has very limited information
about working set so they want to involve memory management more heavily
like android's lowmemory killer and ashmem or recent many lowmemory
notifier.

One of the simple imagine scenario about userspace's intelligence is that
platform can manage tasks as forground and background so it would be
better to reclaim background's task pages for end-user's *responsibility*
although it has frequent referenced pages.

This patch adds new knob "reclaim under proc/<pid>/" so task manager
can reclaim any target process anytime, anywhere. It could give another
method to platform for using memory efficiently.

It can avoid process killing for getting free memory, which was really
terrible experience because I lost my best score of game I had ever
after I switch the phone call while I enjoyed the game.

Reclaim file-backed pages only.
	echo file > /proc/PID/reclaim
Reclaim anonymous pages only.
	echo anon > /proc/PID/reclaim
Reclaim all pages
	echo all > /proc/PID/reclaim

Bug: 121158602
Bug: 122047783
Change-Id: I2f629f7a43289af114df27044b1d2af4a6e785bc
Signed-off-by: Tim Murray <timmurray@google.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
(cherry picked from commit e48b0dd9cc1fa572455434357f5678018b3458b9)
modification for zone-lru list from 4.9