memcg: implement memory thresholds

It allows to register multiple memory and memsw thresholds and gets
notifications when it crosses.

To register a threshold application need:
- create an eventfd;
- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
- write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
  cgroup.event_control.

Application will be notified through eventfd when memory usage crosses
threshold in any direction.

It's applicable for root and non-root cgroup.

It uses stats to track memory usage, simmilar to soft limits. It checks
if we need to send event to userspace on every 100 page in/out. I guess
it's good compromise between performance and accuracy of thresholds.

[[email protected]: coding-style fixes]
[[email protected]: fix documentation merge issue]
Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Dan Malek <[email protected]>
Cc: Vladislav Buzov <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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