cluster/dht: Fix dict_t leaks in rebalance process' execution path

Two dict_t objects are leaked for every file migrated in success codepath.
It is the caller's responsibility to unref dict that it gets from calls to
syncop_getxattr(); and rebalance performs two syncop_getxattr()s per file
without freeing them.

Also, syncop_getxattr() on GF_XATTR_LINKINFO_KEY doesn't seem to be using
the response dict. Hence, NULL is now passed as opposed to @dict to
syncop_getxattr().

Change-Id: I5a4b5ab834df3633dea994f239bbdbc34cbe9259
BUG: 1142052
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/8763
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>