LU-14172 lmv: optimize dir shard revalidate

LU-14172 lmv: optimize dir shard revalidate

mdt_is_remote_object() will check whether child is directory shard
if parent and child are on different MDTs, which needs to read LMV
from disk, and hurt striped directory stat performance.

This can be optimized, client can just set CROSS_REF flag to do a
cross reference getattr, which avoids lots of checks.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ib2d5a510b27c90a26f979f9cccfd40948e32d91a
Reviewed-on: https://review.whamcloud.com/40863
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>