LU-7074 mdd: validate the linkea before packing

LU-7074 mdd: validate the linkea before packing

During migration, let's validate linkea entry before
packing the updates into the buffer and sending to
the remote MDT.

And also move retrieving linkea before transaction
start to avoiding sending RPC inside the transaction.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I36f235274d39560f6654fd76967e45400e8187ce
Reviewed-on: http://review.whamcloud.com/16235
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>