Do not assume sizeof(size_t)

This fixes an assumption that sizeof(size_t) == sizeof(uint64_t), which
is not guaranteed. At least on NetBSD/i386, size_t is 32 bit long.

Caught by tests/basics/file-snapshot.t

BUG: 764655
Change-Id: Ib7620a2ffe8758521886af37bc280101a040d860
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8441
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Dan Lambright <dlambrig@redhat.com>