cluster/dht: fix memory corruption in locking api.

<man 3 qsort>

     The  contents  of the array are sorted in ascending order
     according to a comparison function pointed to by compar, which is
     called with two arguments that "point to the objects being
     compared".

</man 3 qsort>

qsort passes "pointers to members of the array" to comparision
function. Since the members of the array happen to be (dht_lock_t *),
the arguments passed to dht_lock_request_cmp are of type (dht_lock_t
**). Previously we assumed them to be of type (dht_lock_t *), which
resulted in memory corruption.

Change-Id: Iee0758704434beaff3c3a1ad48d549cbdc9e1c96
BUG: 1139506
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/8659
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>