LU-3200 mdc: layout lock rpc must not take rpc_lock

LU-3200 mdc: layout lock rpc must not take rpc_lock

When a client issue an RPC to get a layout lock, it
must not hold rpc_lock because in case of a restore
the rpc can be blocking for a long time

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: Id9ea9be47669fd52c8d6e681090b4c911cfe2dc1
Reviewed-on: http://review.whamcloud.com/6115
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>