UPSTREAM: cgroup: implement __cgroup_task_count() helper

The helper is identical to the existing cgroup_task_count()
except it doesn't take the css_set_lock by itself, assuming
that the caller does.

Also, move cgroup_task_count() implementation into
kernel/cgroup/cgroup.c, as there is nothing specific to cgroup v1.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
Change-Id: Iaa9085d2375d395a051543d2555389213c2892d6
(cherry picked from commit aade7f9efba098859681f8e88d81a5b44ad09b12)
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>