[BUGFIX][PATCH v6] memcg: fix percpu cached charge drainingfrequency

[KAMEZAWA Hiroyuki]

This is a repleacement for
memcg-fix-percpu-cached-charge-draining-frequency.patch
+
memcg-fix-percpu-cached-charge-draining-frequency-fix.patch


Changelog:
  - removed unnecessary rcu_read_lock()
  - removed a fix for softlimit case (move to another independent patch)
  - make mutex static.
  - applied comment updates from Andrew Morton.

A patch for softlimit will follow this.

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