From: Alex Tomas <bzzz@tmi.comex.ru>

This is the second half of the vm_enough_memory() speedup.

When overcommit_memory != 1, vm_enough_memory() calls get_page_state() to
calculate the amount of used pagecache.  It does this on every call to
sys_brk().

get_page_state() is really expensive on SMP.

So the patch arranges for pagecache accounting to be in a global atomic_t,
with per-cpu batching and approximate accounting to amortise the cost of the
global atomic.

The nr_pagecache field of /proc/vmstat is removed.