MB-6657: off_t ==> cs_off_t

Change-Id: If81d5751a26d73e0d5fb9670b647693bdfaa33e3
Reviewed-on: http://review.couchbase.org/25284
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Sriram Melkote <siri.couchbase@gmail.com>