LU-4471 mdd: mdd_unlink: do trans_start after sanity check

Currently, mdd_trans_start is called before
mdd_unlink_sanity_check. This means a remote directory
which has files in it can be removed on MDT0 before the
sanity check on MDT1 finds the files and errors, which
orphans the files on MDT1. This patch moves the sanity
check before mdd_trans_create and mdd_trans_start.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I08882b682b9f0016577214821efec4759ee5c184
Reviewed-on: http://review.whamcloud.com/8827
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>