Use afs_foff_t for file offsets

Use afs_foff_t for file offsets

Some callers in vol and volser were not using afs_foff_t for file
offsets for calls to STREAM_ASEEK, FDH_TRUNC, FDH_PREAD, or
FDH_PWRITE. Most notably, in code for volume cloning, purging, and
dumping operations. Fix them to use afs_foff_t to prevent errors when
dealing with large special files.

Change-Id: I531fa8cb3070007a9c03e5e19c70d546f272a12c
Reviewed-on: http://gerrit.openafs.org/3289
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>