cgroup: Remove task_lock() from cgroup_post_fork()

cgroup_post_fork() is protected between threadgroup_change_begin()
and threadgroup_change_end() against concurrent changes of the
child's css_set in cgroup_task_migrate(). Also the child can't
exit and call cgroup_exit() at this stage, this means it's css_set
can't be changed with init_css_set concurrently.

For these reasons, we don't need to hold task_lock() on the child
because it's css_set can only remain stable in this place.

Let's remove the lock there.

v2: Update comment to explain that we are safe against
cgroup_exit()

Signed-off-by: Frederic Weisbecker <[email protected]>
Acked-by: Li Zefan <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
Cc: Containers <[email protected]>
Cc: Cgroups <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: Mandeep Singh Baines <[email protected]>

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