memcgroup: move memory controller allocations to their own slabs

Move the memory controller data structure page_cgroup to its own slab cache.
It saves space on the system, allocations are not necessarily pushed to order
of 2 and should provide performance benefits.  Users who disable the memory
controller can also double check that the memory controller is not allocating
page_cgroup's.

NOTE: Hugh Dickins brought up the issue of whether we want to mark page_cgroup
as __GFP_MOVABLE or __GFP_RECLAIMABLE.  I don't think there is an easy answer
at the moment.  page_cgroup's are associated with user pages, they can be
reclaimed once the user page has been reclaimed, so it might make sense to
mark them as __GFP_RECLAIMABLE.  For now, I am leaving the marking to default
values that the slab allocator uses.

Signed-off-by: Balbir Singh <[email protected]>
Cc: Pavel Emelianov <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Sudhir Kumar <[email protected]>
Cc: YAMAMOTO Takashi <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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