sched/fair: Cleanup loop_max and loop_break

sched_nr_migrate_break is set to a fix value and never changes so we can
replace it by a define SCHED_NR_MIGRATE_BREAK.

Also, we adjust SCHED_NR_MIGRATE_BREAK to be aligned with the init value
of sysctl_sched_nr_migrate which can be init to different values.

Then, use SCHED_NR_MIGRATE_BREAK to init sysctl_sched_nr_migrate.

The behavior stays unchanged unless you modify sysctl_sched_nr_migrate
trough debugfs.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220825122726.20819-3-vincent.guittot@linaro.org