sched: Move sched_autogroup_exit() to free_signal_struct()

Per Oleg's suggestion, undo fork failure free/put_signal_struct change,
and move sched_autogroup_exit() to free_signal_struct() instead.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1294222564.8369.6.camel@marge.simson.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>