sched/fair: Encapsulate set custom slice in a __setparam_fair() function

Similarly to dl, create a __setparam_fair() function to set parameters
related to fair class and move it in the fair.c file.

No functional changes expected

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Link: https://lore.kernel.org/r/20250110144656.484601-1-vincent.guittot@linaro.org