A shutdown or unmount of AFS on OpenBSD will invariably result in a kernel
panic. This is because the afs_unmount() routine does not (can not?) force
vnode releases if the vnode is still busy. However, it continues on
nonetheless and dies a horrible death a little later.
This update causes a return from afs_unmount() with EBUSY if all the vnodes
weren't released. This results in error messages on shutdown but the overall
process continues more reliably and reboots, for example, work.
There is likely a better solution to this but at least this is no worse than
a system crash and it doesn't require console (or power button) intervention
so it should do until I have the chance to explore further.