cluster/dht: Do not open fd in migration check/complete for non fd ops

if local->fd == NULL, then in dht_migration_check_complete, do not do
open call. Let the layout get updated, and proceed with invoking the
registered target_fn.

if local->fd == NULL, do not call dht_rebalance_in_progress_check for
truncate fop, but proceed with truncate2.

Change-Id: Ia5a5d40bcea7bfb320ef7096af1e035b8847d4ff
BUG: 960055
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4958
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>