staging/lustre/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

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3200
Lustre-change: http://review.whamcloud.com/6115


Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>