panic from interrupt stack loses

[rws%mit-bold at sri-unix.UUCP]

From:  Robert W. Scheifler <rws at mit-bold>

Description:
	Despite my earlier bug fix for the spl1() in boot(), calling
	panic while executing on the interrupt stack still loses,
	because update() gets called, and eventually biowait(), which
	calls sleep(), which does an spl0().  A (disk) interrupt
	will then eventually cause an REI back on to the interrupt
	stack at IPL 0, which is illegal, causing a
	panic: Reserved operand.
Repeat-By:
	Cause a panic in an interrupt service routine while there
	are outstanding disk writes.
Fix:
	There is no easy fix as far as I can see.
	It might be better if such panics didn't try to update()?