cgroup: Drop task_lock(parent) on cgroup_fork()

cgroup: Drop task_lock(parent) on cgroup_fork()

We don't need to hold the parent task_lock() on the
parent in cgroup_fork() because we are already synchronized
against the two places that may change the parent css_set
concurrently:

- cgroup_exit(), but the parent obviously can't exit concurrently
- cgroup migration: we are synchronized against threadgroup_lock()

So we can safely remove the task_lock() there.

Signed-off-by: Frederic Weisbecker <[email protected]>
Reviewed-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]>