LU-6581 mdt: Transfer layout only if layout lock is granted

LU-6581 mdt: Transfer layout only if layout lock is granted

Make sure that only valid layout is transferred;
Client also checks if lock is granted before trusting the layout;
Restore change LU-3299 commit 03a6a8b1 because it breaks the
assumption that l_lvb_data is immutable once assigned;
Comment fixups.

Signed-off-by: wang di <di.wang@intel.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: If18e022131c5015e40e0408cdd0168df237e4338
Reviewed-on: http://review.whamcloud.com/14726
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>