sched/fair: improve fairness efficiency

Measured using hackbench:

PRE-patch :      4.542422684 seconds time elapsed   ( +-  47.40% )
POST-page :      2.389409991 seconds time elapsed   ( +-  47.40% )

Which shows a significant improvement of ~47.40%

Signed-off-by: Paul Reioux <reioux@gmail.com>