vol: Check for blank vnode in VAllocVnode_r

vol: Check for blank vnode in VAllocVnode_r

When we alloc a vnode in VAllocVnode_r, we look up that vnode in the
vnode cache, to see if a vnode struct already exists for it. If it
doesn't, we check the vnode index to ensure that the vnode actually is
not in use (among other things). However, we do not perform the same
check for a vnode already in the cache. Add this check, to make sure
that we don't allocate an already-used vnode number, even if the
bitmap is screwed up.

Reviewed-on: http://gerrit.openafs.org/4060
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 974e95302a312c03cd735c0fdaa67fb02d254e26)

Change-Id: Ie75dc32c2a2c1ff05a96a1a14650e41ada07609d
Reviewed-on: http://gerrit.openafs.org/4170
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>