cgroups: add res counter common ancestor searching

cgroups: add res counter common ancestor searching

Add a new API to find the common ancestor between two resource counters.
This includes the passed resource counter themselves.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <paul@paulmenage.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Aditya Kali <adityakali@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Tim Hockin <thockin@hockin.org>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>