igmp_setup(), which `igmp off` also calls, enabled the lookup of IPv4
multicast in the L2 table. Groups learned while IGMP was on therefore
kept going only to the ports that had reported them, and with reports
no longer trapped nothing updated those entries until a reboot.
The lookup is now enabled in igmp_enable() and disabled in igmp_setup(),
so with IGMP off all IP multicast is flooded again.
DrDoof
aba58e7f99