sched: Add sysctl_sched_shares_window

Introduce a new sysctl for the shares window and disambiguate it from
sched_time_avg.

A 10ms window appears to be a good compromise between accuracy and performance.

Signed-off-by: Paul Turner <pjt@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20101115234938.112173964@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>