sched: revert stable c6fc81a sched: Fix a race between ttwu() and migrate_task()

sched: revert stable c6fc81a sched: Fix a race between ttwu() and migrate_task()

This commit does not appear to have been meant for 32-stable, and causes ltp's
cpusets testcases to fail, revert it.

Original commit text:

sched: Fix a race between ttwu() and migrate_task()

Based on commit e2912009fb7b715728311b0d8fe327a1432b3f79 upstream, but
done differently as this issue is not present in .33 or .34 kernels due
to rework in this area.

If a task is in the TASK_WAITING state, then try_to_wake_up() is working
on it, and it will place it on the correct cpu.

This commit ensures that neither migrate_task() nor __migrate_task()
calls set_task_cpu(p) while p is in the TASK_WAKING state.  Otherwise,
there could be two concurrent calls to set_task_cpu(p), resulting in
the task's cfs_rq being inconsistent with its cpu.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>