rcu: Use a new sched_set_rcu_fifo() to set gp kthread prio

sched_setscheduler() is considsered a private function now. To
facilitate the transition we need to move existing users to a better
API.

RCU sets the priority of its grace period kthread via kernel parameter.
This is a special case, hence create a special function that uses this
kernel parameter to convert a task to fifo.

It is meant to be singly used by RCU.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>