MAINTAINERS: make Vladimir co-maintainer of the memory controller

Vladimir architected and authored much of the current state of the
memcg's slab memory accounting and tracking.  Make sure he gets CC'd on
bug reports ;-)

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>