sched, autogroup: Stop going ahead if autogroup is disabled

sched, autogroup: Stop going ahead if autogroup is disabled

when autogroup is disable from the beginning,
sched_autogroup_create_attach()
  autogroup_move_group()                    <== 1
    sched_move_task()                       <== 2
      task_move_group_fair()
        set_task_rq()
          task_group()
            autogroup_task_group()

We go the whole path without doing anything useful.

Then stop going further if autogroup is disabled.

But there will be a race window between 1 and 2, in which
sysctl_sched_autogroup_enabled is enabled. This issue
will be toke by following patch.

Signed-off-by: Yong Zhang <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Mike Galbraith <[email protected]>
LKML-Reference: <1298185696[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>