unlocked vnode when trying to rename a directory. The fix was to
shuffle some bits around and #pray.
The rename routine actually needs a very very major wide-angle whopping:
* it takes locks out-of-order
* it deals with references from SAVESTART lookups in interesting ways
* I doubt there is any guarantee for correct operation if there
are multiple concurrent accesses
* the error branches might just as well call panic() directly
4a0a4d4f30