fix dns_qp_insert() checks in qpzone

fix dns_qp_insert() checks in qpzone

in some places there were checks for failures of dns_qp_insert()
after dns_qp_getname(). such failures could only happen if another
thread inserted a node between the two calls, and that can't happen
because the calls are serialized with dns_qpmulti_write(). we can
simplify the code and just add an INSIST.