nfs3-server: call truncate fop only if necessary

* nfs3svc_setattr_cbk: only truncate if requested size != current size

Change-Id: I3d89e4d2b0710118f90cf5bf9cfdea61d8877491
BUG: 960725
Signed-off-by: Michael Brown <michael@netdirect.ca>
Reviewed-on: http://review.gluster.org/4917
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>