OpenBSD: allow for more graceful shutdown

OpenBSD: allow for more graceful shutdown

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.

Change-Id: Ia70f83bda748ea3d0b81b341a292e83121446567
Reviewed-on: http://gerrit.openafs.org/1275
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>