DHT/Rebalance : Hard link Migration Failure

Probelm : __is_file_migratable used to return ENOTSUP
for all the cases. Hence, it will add to the failure
count. And the remove-brick status will show failure
for all the files.

Solution : Added 'ret = -2' to gf_defrag_handle_hardlink to
be deemed as success. Otherwise dht_migrate_file will try to
migrate each of the hard link, which not intended.

Change-Id: Iff74f6634fb64e4b91fc5d016e87ff1290b7a0d6
BUG: 1066798
Signed-off-by: Susant Palai <spalai@redhat.com>
Reviewed-on: http://review.gluster.org/7124
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>