ipv6: mld: answer mldv2 queries with mldv1 reports in mldv1 fallback

RFC2710 (MLDv1), section 3.7. says:

  The length of a received MLD message is computed by taking the
  IPv6 Payload Length value and subtracting the length of any IPv6
  extension headers present between the IPv6 header and the MLD
  message. If that length is greater than 24 octets, that indicates
  that there are other fields present *beyond* the fields described
  above, perhaps belonging to a *future backwards-compatible* version
  of MLD. An implementation of the version of MLD specified in this
  document *MUST NOT* send an MLD message longer than 24 octets and
  MUST ignore anything past the first 24 octets of a received MLD
  message.

RFC3810 (MLDv2), section 8.2.1. states for *listeners* regarding
presence of MLDv1 routers:

  In order to be compatible with MLDv1 routers, MLDv2 hosts MUST
  operate in version 1 compatibility mode. [...] When Host
  Compatibility Mode is MLDv2, a host acts using the MLDv2 protocol
  on that int...