cgroup: convert memcg controller to the new cftype interface

Convert memcg to use the new cftype based interface.  kmem support
abuses ->populate() for mem_cgroup_sockets_init() so it can't be
removed at the moment.

tcp_memcontrol is updated so that tcp_files[] is registered via a
__initcall.  This change also allows removing the forward declaration
of tcp_files[].  Removed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Greg Thelen <gthelen@google.com>