memcg : share event counter rather than duplicate

Memcg has 2 eventcountes which counts "the same" event.  Just usages are
different from each other.  This patch tries to reduce event counter.

Now logic uses "only increment, no reset" counter and masks for each
checks.  Softlimit chesk was done per 1000 evetns.  So, the similar check
can be done by !(new_counter & 0x3ff).  Threshold check was done per 100
events.  So, the similar check can be done by (!new_counter & 0x7f)

ALL event checks are done right after EVENT percpu counter is updated.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>