salvager: Fix volume parsing on 64-bit

salvager: Fix volume parsing on 64-bit

When an unsigned long is wider than an afs_uint32, comparing the
afs_uint32 vid to ULONG_MAX is always going to be false (which the
compiler can warn us about). Fix this by storing to an unsigned long,
and converting to a volume id after ensuring that the result is not
too large.

Change-Id: Ifbd724dabd988bc4b1ba6ee8f3dc7fa1a0afb226
Reviewed-on: http://gerrit.openafs.org/4244
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>