vol: remove potential data loss warnings in vol-salvage.c

vol: remove potential data loss warnings in vol-salvage.c

afs_sfsize_t is not always the same size as 'int'.
Avoid warnings by using afs_sfsize_t in the appropriate locations.

Change-Id: If77379b17fa984e3dacd2740726b7309a0e640dd
Reviewed-on: http://gerrit.openafs.org/3706
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-on: http://gerrit.openafs.org/3853