Make file offsets in vldb layout unsigned ints

Make file offsets in vldb layout unsigned ints

vlserver.h has a few cases where links through the vldb are being
stored as afs_int32s. These are file offsets and should be unsigned.
This commit fixes just these declarations.  The code still compiles
(because they are silently cast all over the shop), but it starts
the tidy effort.

Change-Id: Iba14bd28cb43a6ca6401be4252bb45135d8b3a55
Reviewed-on: http://gerrit.openafs.org/2027
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>