replace preempt_thresh with a set of knobs to control preemption for each priority class

[avg]

Summary

If preemption is enabled the new code always allows a thread from a
"higher" class to preempt a thread of a "lower" class. For example, a
real-time thread can always preempt a kernel thread or a timeshare
thread. The knobs allow to tune whether threads from the same class can
preempt each other (given that the priorities permit for that).