cluster/dht: Don't rely on linkto xattr to find destination subvol

during phase 2 of migration.

linkto xattr on source file cannot be relied to find where the data
file currently resides. This can happen if there are multiple
migrations before phase 2 detection by a client. For eg.,

* migration (M1, node1, node2) starts.
* application writes some data. DHT correctly stores the state in
  inode context that phase-1 of migration is in progress
* migration M1 completes
* migration (M2, node2, node3) is triggered and completed
* application resumes writes to the file. DHT identifies it as phase-2
  of migration. However, linkto xattr on node1 points to node2, but
  the file is on node3. A lookup correctly identifies node3 as cached
  subvol

TBD:
   When we identify phase-2 of a previous migration (say M1), there
   might be a migration in progress - say (M3, node3, node4). In this
   case we need to send writes to both (node3, node4) not just
   node3. Also, the inode state needs to correctly indicate that its in
   phase-1 of migration. I'll send this as a different patch.

Change-Id: I1a861f766258170af2f6c0935468edb6be687b95
BUG: 1225809
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/10805
Reviewed-on: http://review.gluster.org/10965
Tested-by: Gluster Build System <jenkins@build.gluster.com>

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