LU-11631 mdd: migrate symlink for cross-MDT rename

LU-11631 mdd: migrate symlink for cross-MDT rename

If symlink rename is cross MDTs, it's ineffective to turn this
symlink into a remote object, instead migrate it to where target
MDT is. The following changes are made:
* change migration code to allow source and target have different
  name.
* if symlink is renamed to other MDT, and it doesn't have other
  hard link and target doesn't exist, migrate it to target MDT.
* remove mdd_rename_order() which is obsolete.

Add sanity 24G.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ib5fafe3122172ac582bbcc907c72a9f391baf0e1
Reviewed-on: https://review.whamcloud.com/39897
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>