mm: page-writeback: fix page state calculation in throttle_vm_writeout

It was found that a number of tasks were blocked in the reclaim path
(throttle_vm_writeout) for seconds, because of vmstat_diff not being
synced in time. Fix that by adding a new function
global_page_state_snapshot.

Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Change-Id: Iec167635ad724a55c27bdbd49eb8686e7857216c