mds: Fix straydn race between unlink/rename linkmerge

mds: Fix straydn race between unlink/rename linkmerge

Handle race between unlink of secondary hardlink (say hl_file1) and
linkmerge rename, triggered by unlink of primary (say file1) on to
the same secondary hardlink (hl_file1).
           1. ln file1 hl_file1
           2. unlink hl_file1
           3. prepares straydn for referent inode
           4. wait for the locks

   unlink file1, triggers linkmerge rename
   rename(hl_file1, primary_stray) ---> referent straydn converted to primary

           5. On retry, it find itself as primary,
              needs to prepare straydn for primary.
              so clean up the earlier referent straydn

Fixes: https://tracker.ceph.com/issues/54205
Signed-off-by: Kotresh HR <khiremat@redhat.com>