cluster/dht: update ctx-time only if we receive the new iatt

1. Used local->postparent(contains merged iatt of all succesful calls) instead
of postparent for dht ctx time update.

2. dht_inode_ctx_time_update avoided in case of opret -1.

Change-Id: Ie04a7842a41c241f911b6a3f76267b996d27fb43
BUG: 881013
Signed-off-by: Varun Shastry <vshastry@redhat.com>
Reviewed-on: http://review.gluster.org/4338
Reviewed-by: Shishir Gowda <sgowda@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>