MB-6657: cs_off_t should be a signed variable

off_t is supposed to be signed

Change-Id: I7c30bea3d534cd01b2d576815f0841bd96e66c23
Reviewed-on: http://review.couchbase.org/25282
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>