handlers if interrupts are nested more than a few (3) deep. This
only reduces the maximum nesting level by 1 with the standard
drivers unless there is a related bug somewhere, but can't hurt
much (the worst case is returning to hoggish interrupt handler like
wdintr(), but such interrupt handlers hurt anyway).
Fixed a previously harmless race incrementing the interrupt nesting
level.
This should be in 2.1.6 and 2.2.