sched: Provide idle_rq() helper

sched: Provide idle_rq() helper

A fix for the dl_server 'requires' idle_cpu() usage, which made me
note that it and available_idle_cpu() are extern function calls.

And while idle_cpu() is used outside of kernel/sched/,
available_idle_cpu() is not.

This makes it hard to make idle_cpu() an inline helper, so provide
idle_rq() and implement idle_cpu() and available_idle_cpu() using
that.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>