mcast: fix leaked igmp packets on multicast cleanup

When we release a multicast address (e.g. on interface shutdown) we may
still have packets queued in inm_scq. We have to free those, or we'll
leak memory.

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43033