memcg: use native word page statistics counters

memcg: use native word page statistics counters

The statistic counters are in units of pages, there is no reason to make
them 64-bit wide on 32-bit machines.

Make them native words.  Since they are signed, this leaves 31 bit on
32-bit machines, which can represent roughly 8TB assuming a page size of
4k.

[[email protected]: coding-style fixes]
Signed-off-by: Johannes Weiner <[email protected]>
Signed-off-by: Greg Thelen <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
Acked-by: Balbir Singh <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>