LU-10192 lfsck: verify agent entry

LU-10192 lfsck: verify agent entry

Originally, we only support agent entry for ldiskfs backend,
and the osd-ldiskfs will maintain agent entry from cross-MDTs
directory, NOT corss-MDTs regular file. So if someone create
cross-MDTs hard link or renames regular file cross-MDTs, then
related object will become invisible to userspace when mount
the MDT as 'ldiskfs' directly.

On the other hand, old ZFS based MDT also did not support
agent entry. When upgraded from the old ZFS based device,
or migrated from old ldiskfs based MDT to new ZFS based MDT,
some (or all) agent entries need to be created.

So we enhance the namespace LFSCK logic to check whether the
agent entry is properly setup or not. If not, the LFSCK will
trigger the lower layer agent entry verify mechanism via set
xattr operation.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I0aa83aff8b39b894dbde19f573c078faf0ef249c
Reviewed-on: https://review.whamcloud.com/29985
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>