mcast: fix memory leak in imf_purge()

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")
Differential Revision:	https://reviews.freebsd.org/D41629

sys/netinet/in_mcast.c

@@ -915,6 +915,8 @@ imf_purge(struct in_mfilter *imf)
 	imf->imf_st[0] = imf->imf_st[1] = MCAST_UNDEFINED;
 	KASSERT(RB_EMPTY(&imf->imf_sources),
 	    ("%s: imf_sources not empty", __func__));
+	if (imf->imf_inm != NULL)
+		mbufq_drain(&imf->imf_inm->inm_scq);
 }
 
 /*