LU-5509 osd: get PFID from linkEA for remote dir on ldiskfs

LU-5509 osd: get PFID from linkEA for remote dir on ldiskfs

On the ldiskfs backend, for a directory which parent resides on
remote MDT, to satisfy the local e2fsck, we insert it into the
/REMOTE_PARENT_DIR locally. On the other hand, to make the lookup(..)
on the directory can return the real parent FID, we append the real
parent FID after its ".." name entry in the /REMOTE_PARENT_DIR.

Unfortunately, such PFID-in-dirent cannot be preserved via file-level
backup. So after the restore, we cannot get the right parent FID from
its ".." name entry in the /REMOTE_PARENT_DIR. Under such case, since
we have stored the real parent FID in the directory object's linkEA,
we can parse the linkEA for the real parent FID.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Icf1e24ec911818b3a49a253f67c72334a4b75712
Reviewed-on: http://review.whamcloud.com/11485
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>