memcgroup: tidy up mem_cgroup_charge_common

memcgroup: tidy up mem_cgroup_charge_common

Tidy up mem_cgroup_charge_common before extending it.  Adjust some comments,
but mainly clean up its loop: I've an aversion to loops full of continues,
then a break or a goto at the bottom.  And the is_atomic test should be on the
__GFP_WAIT bit, not GFP_ATOMIC bits.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>