cluster/dht: invoke callback when there are no locks to be unlocked.

Change-Id: I375cb68f1075c2d58cf9d09ed6bd5e2746e1637d
BUG: 1130888
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/8549
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: N Balachandran <nbalacha@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>