cgroup: reorganize cgroup.procs / task write path

Currently, writes "cgroup.procs" and "cgroup.tasks" files are all
handled by __cgroup_procs_write() on both v1 and v2.  This patch
reoragnizes the write path so that there are common helper functions
that different write paths use.

While this somewhat increases LOC, the different paths are no longer
intertwined and each path has more flexibility to implement different
behaviors which will be necessary for the planned v2 thread support.

v3: - Restructured so that cgroup_procs_write_permission() takes
      @src_cgrp and @dst_cgrp.

v2: - Rolled in Waiman's task reference count fix.
    - Updated on top of nsdelegate changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Waiman Long <longman@redhat.com>