- Since critical sections no longer raise the processor interrupt level to

  above what's used for fast interrupts, only interrupts with the level of
  the interrupt which led to calling intr_fast() (which is used with both
  fast and ithread interrupts) are blocked while in that function. Thus
  intr_fast() can be preempted by a fast interrupt (which are of a higher
  level than ithread interrupts) while servicing an ithread interrupt. This
  can lead to a stale pointer to the head of the active interrupt requests
  list when back in the ithread interrupt invocation of intr_fast(), in turn
  resulting in corruption of the interrupt request lists and consequently
  in a panic. Solve this be turning off interrupts in intr_fast() before
  reading the pointer to the head of the active list rather than after. [1]
- Add a KASSERT in intr_fast() which asserts that ir_func is non-zero before
  calling it. [1]
- Increment interrupt stats after calling the handlers rather than before.
  This reduces the delay until direct and fast handlers are serviced, in my
  testings by 30% on average for the direct tick interrupt handler, in turn
  resulting in less clock drift.

PR:		94778 [1]
Submitted by:	Andrew Belashov [1]
MFC after:	2 weeks

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