memcg: make mem_cgroup_split_huge_fixup() more efficient

In split_huge_page(), mem_cgroup_split_huge_fixup() is called to handle
page_cgroup modifcations.  It takes move_lock_page_cgroup() and modifies
page_cgroup and LRU accounting jobs and called HPAGE_PMD_SIZE - 1 times.

But thinking again,
  - compound_lock() is held at move_accout...then, it's not necessary
    to take move_lock_page_cgroup().
  - LRU is locked and all tail pages will go into the same LRU as
    head is now on.
  - page_cgroup is contiguous in huge page range.

This patch fixes mem_cgroup_split_huge_fixup() as to be called once per
hugepage and reduce costs for spliting.

[akpm@linux-foundation.org: fix typo, per Michal]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>