Restore the td_cookie value for the tmpfs directory entry which was a
dup entry, upon detach from the parent directory. If the node is
renamed, the entry is re-attached at the different directory, and
invalud cookie value triggers assert (or corrupts directory rb tree,
it seems).

Reported by: clusteradm (gjb, antoine)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week