cgroup: relocate cftype and cgroup_subsys definitions in controllers

blk-cgroup, netprio_cgroup, cls_cgroup and tcp_memcontrol
unnecessarily define cftype array and cgroup_subsys structures at the
top of the file, which is unconventional and necessiates forward
declaration of methods.

This patch relocates those below the definitions of the methods and
removes the forward declarations.  Note that forward declaration of
tcp_files[] is added in tcp_memcontrol.c for tcp_init_cgroup().  This
will be removed soon by another patch.

This patch doesn't introduce any functional change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>