LU-17284 mdt: revalidate object for migration

LU-17284 mdt: revalidate object for migration

if the source object is remote, then we should revlidate it
once the object's ldlm lock is granted. otherwise we can't
use the object's attributes:
lu_object_attr())
	ASSERTION( ((o)->lo_header->loh_attr & LOHA_EXISTS) != 0

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I9896cdd011f858091ac68b50b74e2f1f027f7331
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53087
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>