panic in sbappendaddr_locked

[Justin Hibbits]

A recent update -- March 11 and March 13 both -- causes a panic in
sbappendaddr_locked on powerpc.  Kernel is a modified GENERIC.  I can't get a
backtrace because the keyboard doesn't work in the debugger, so the best I can
do is say that the kernel is modified GENERIC without INVARIANTS and WITNESS,
and with sound and snd_ai2s.  The last-known-good kernel that I have is from
February 28.

- Justin