MFC r273855:
  Fix mbuf leak in IPv6 multicast code.
  When multicast capable interface goes away, it leaves multicast groups,
  this leads to generate MLD reports, but MLD code does deffered send and
  MLD reports are queued in the in6_multi's in6m_scq ifq. The problem is
  that in6_multi structures are freed when interface leaves multicast groups
  and thread that does deffered send will not take these queued packets.

  PR:		194577

MFC r273857:
  Move ifq drain into in6m_purge().

  Suggested by:	bms
Sponsored by:	Yandex LLC