staging: lustre: 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, if it did not get
   the layout lock.

Signed-off-by: wang di <di.wang@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4682
Reviewed-on: http://review.whamcloud.com/9522


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>