sched/fair: Fair server interface

Add an interface for fair server setup on debugfs.

Each CPU has two files under /debug/sched/fair_server/cpu{ID}:

 - runtime: set runtime in ns
 - period:  set period in ns

This then leaves /proc/sys/kernel/sched_rt_{period,runtime}_us to set
bounds on admission control.

The interface also add the server to the dl bandwidth accounting.

Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/a9ef9fc69bcedb44bddc9bc34f2b313296052819.1716811044.git.bristot@kernel.org