client: writev,fsync to use correct rsp structure

Presently writev_cbk and fsync_cbk pass truncate_rsp for decoding, this
should not create any problems as they are structurally the same. Should
they diverge in the future this could show up as a bug.

Change-Id: Id7da7b6a20f468ca943ceb7926de64b7692f7ec8
BUG: 1164559
Signed-off-by: Rudra Siva <rudrasiva11@gmail.com>
Reviewed-on: http://review.gluster.org/9134
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>