only covers about 3-4 lines.
- Don't lower the IPL while we are on the interrupt stack. Instead, save
the raised IPL and change the saved IPL in sched_lock to IPL_0 before
calling mi_switch(). When we are resumed, restore the saved IPL in
sched_lock to the saved raised IPL so that when we release sched_lock
we won't lower the IPL. Without this, we would get nested interrupts
that would overflow the kernel stack.
Tested by: mjacob