mcast: fix memory leak in imf_purge()

[kp]

Summary

The IGMP code buffers packets in the imf_inm->inm_scq mbufq, but does
not clear this queue when struct in_mfilter is freed by imf_purge().
This can cause memory leaks if IGMPv3 is used.

Purge the mbufq on imf_purge().

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

Test Plan

Running this in a loop shows a steady increate in the mbuf zone, which goes away with this fix:

epair=$(ifconfig epair create)
epair=${epair%a}

ifconfig ${epair}a up
ifconfig ${epair}b up
ifconfig ${epair}a vhid 2 advbase 2 advskew 100 pass mekmitasdigoat alias 192.168.2.1/24

sleep 1
ifconfig ${epair}a destroy