LU-7091 mdt: release cross-MDT lock immediately

LU-7091 mdt: release cross-MDT lock immediately

Because the cross-MDT operations are relatively rare
compared with the normal operation, so let's release
cross-MDT lock immediately after the operation.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I48be60a13e9d10a92595c7faeb91dd8c106b2d42
Reviewed-on: http://review.whamcloud.com/16372
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>