cluster/dht: Fix anomaly check

We were wrongly detecting holes/overlaps for already accounted
errors. Additionally, sort should also handle zero'ed out layout

Change-Id: Ic3d13e1d735b914f9acc01fe919bc90656baea48
BUG: 1003851
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/5762
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/6469
Reviewed-by: Vijay Bellur <vbellur@redhat.com>