sched: call update_curr() in task_tick_fair()

update the fair-clock before using it for the key value.

[ mingo@elte.hu: small cleanups. ]

Signed-off-by: Ting Yang <tingy@cs.umass.edu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>