vol: Correct VolumeNumber for large volume IDs

vol: Correct VolumeNumber for large volume IDs

VolumeNumber was using atoi to convert a volume header name to a
volume ID. This can return just -1 for volume IDs larger than 2^31-1,
though, so use strtoul instead.

Change-Id: Ibae2d29f196646154fcc4f6ff174246e37b3d735
Reviewed-on: http://gerrit.openafs.org/4199
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>