Make the ::exit method act like ::attach, it is after all very nearly
the same thing.

The bug had no effect on correctness - fixing it is an optimization for
the scheduler. Also, later perf-cgroups patches rely on it.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: NPaul Menage <menage@google.com>
LKML-Reference: <1297160655.13327.92.camel@laptop>
Signed-off-by: NIngo Molnar <mingo@elte.hu>