also failed to handle the case where it had discovered that a node didn't
exist, had unlocked and relocked the tree (into write mode), and then had
attempted to add the node and got DNS_R_EXISTS as the result.  (I.e. someone
beat us in the race to create the node.)  In this case, an error would have
been returned.  The correct behavior is to succeed (being sure to increment
the node reference count).