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.

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>
(cherry picked from commit ef8c3db90ae9ec25705027c609a1bae5f186c4f1)

Change-Id: I6f950fd23eba9e00470e424ae49ee6ce9d7ca6f4
Reviewed-on: http://gerrit.openafs.org/3618
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>