32 bit fix: use off_t and not size_t for truncate()

Make sure off_t and not size_t is used when holding file offsets for
ftruncate()/truncate(). It works on 64 bit machines where
sizeof(size_t) == sizeof(off_t) == 8, but breaks for big offsets on 32 bit
machines because sizeof(size_t) == 4 and sizeof(off_t) == 8

BUG: 1129939
Change-Id: Ia2637be772ba9b11731d59fdbffbd269f0ff56c8
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8742
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>