- Don't acquire the vnode interlock in _vn_lock() unless no lock type

 - Don't acquire the vnode interlock in _vn_lock() unless no lock type
   is requested.  Handle this case specially before the while loop.
 - Use the held vnode lock to check for VI_DOOMED.  The vnode lock and
   interlock must both be held to set VI_DOOMED so either one held, even
   shared, is sufficient to check it.

No objection by:	kib