memcg: simple stats for memory resource controller

Implement trivial statistics for the memory resource controller.

Signed-off-by: Balaji Rao <balajirrao@gmail.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>