sched/fair: Proportional newidle balance

sched/fair: Proportional newidle balance

Add a randomized algorithm that runs newidle balancing proportional to
its success rate.

This improves schbench significantly:

 6.18-rc4:			2.22 Mrps/s
 6.18-rc4+revert:		2.04 Mrps/s
 6.18-rc4+revert+random:	2.18 Mrps/S

Conversely, per Adam Li this affects SpecJBB slightly, reducing it by 1%:

 6.17:			-6%
 6.17+revert:		 0%
 6.17+revert+random:	-1%

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Chris Mason <clm@meta.com>
Link: https://lkml.kernel.org/r/6825c50d-7fa7-45d8-9b81-c6e7e25738e2@meta.com
Link: https://patch.msgid.link/20251107161739.770122091@infradead.org