Add a dead mount

["J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>]

Currently dead vnodes still reside on the vnodelist of the file system
they have been removed from.  To me it looks much better to create
a "dead mount" that takes dead vnodes until they get freed.

Diff available at http://www.netbsd.org/~hannken/dead-mount.diff

Comments or objections anyone?

--
J. Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig 
(Germany)