Impact: fix bad-interactivity buglet

Fix sched_slice() to emit a sane result whether a task is currently
enqueued or not.
Signed-off-by: NMike Galbraith <efault@gmx.de>
Tested-by: NJayson King <dev@jaysonking.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

 kernel/sched_fair.c |   30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)