memcg: refactor mem_cgroup_resize_limit()

mem_cgroup_resize_limit() and mem_cgroup_resize_memsw_limit() have
identical logics.  Refactor code so we don't need to keep two pieces of
code that does same thing.

Link: http://lkml.kernel.org/r/20180108224238.14583-1-yuzhao@google.com


Signed-off-by: Yu Zhao <yuzhao@google.com>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>