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.

(cherry picked from commit ce5e263b488f8cb85662031ee08eea448dab2d27)
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>
Change-Id: Iec2d640daed1e9c175d17823750f272297f890a7
Reviewed-on: http://gerrit.openafs.org/4283
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>