LU-4682 llite: a few fixes for migration.

LU-4682 llite: a few fixes for migration.

1. Clear the client dentry cache before migrating file/directory
to the remote MDT.

2. Do not return stripe information to client, it did not get
Layout lock.

3. A few cleanups from the review of 6662.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I209079dfa07241412878b30fd02e8152046c95c7
Reviewed-on: http://review.whamcloud.com/9522
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>