salvager: Do not abort on large volume IDs

salvager: Do not abort on large volume IDs

We have already checked that 'vid' is valid; no need to check if it is
negative. Also change vid to be a VolumeId.

This is partially cherry-picked from
0884e9d0fddf2be81abf6468209048331efa8a1e. The commit
4d691ae10903e01db4d6b24a4eb02da536cadf7c is comprised of changes from
both ce5e263b488f8cb85662031ee08eea448dab2d27 and
0884e9d0fddf2be81abf6468209048331efa8a1e, but it missed a few things
from 0884e9d0fddf2be81abf6468209048331efa8a1e. This commit brings in
the rest of the changes from 0884e9d0fddf2be81abf6468209048331efa8a1e.

Change-Id: I8e001bfe81128b2e2214b3b2fa83e4797374022b
Reviewed-on: http://gerrit.openafs.org/6778
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>