Use an explicit symbol for uninitialized vnode types

Avoid trying to get clever with stuffing -1 into an unsigned bitfield,
which causes the value to change and generates a warning from clang.
Just use vNull, which is intended to be used for uninitialized/empty
vnodes.

Change-Id: I5662887e5a68c7e687025d19226f821d8f2d6a09
Reviewed-on: http://gerrit.openafs.org/10701
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>