Repair ALTQ-tagging rules in IPFW which got broken in the last PF

import.  The PF mbuf-tagging support routines changed to link the
allocated tags into the provided mbuf themselves, so the left-over
m_tag_prepend() was trying to add a bogus (usually NULL) tag.

Reviewed by: mlaier
Approved by: re

sys/netinet/ip_fw2.c

@@ -3077,7 +3077,6 @@ do {									\
 				else
 					at->af = AF_LINK;
 				at->hdr = ip;
-				m_tag_prepend(m, mtag);
 				break;
 			}