sched/fair: Rework and comment the group_capacity code

Pull out the group_capacity computation so that we can more clearly
comment its issues.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-az1hl1ya55k361nkeh9bj0yw@git.kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>