in6_if2idlen: treat bridge(4) interfaces like other Ethernet interfaces

bridge(4) interfaces have an if_type of IFT_BRIDGE, rather than
IFT_ETHER, even though they only support Ethernet-style links.  This
caused in6_if2idlen to emit an "unknown link type (209)" warning to
the console every time it was called.  Add IFT_BRIDGE to the case
statement in the appropriate place, indicating that it uses the same
IPv6 address format as other Ethernet-like interfaces.

MFC after:	1 week

sys/netinet6/in6.c

@@ -2008,6 +2008,7 @@ in6_if2idlen(struct ifnet *ifp)
 	case IFT_PROPVIRTUAL:	/* XXX: no RFC. treat it as ether */
 	case IFT_L2VLAN:	/* ditto */
 	case IFT_IEEE80211:	/* ditto */
+	case IFT_BRIDGE:	/* bridge(4) only does Ethernet-like links */
 	case IFT_INFINIBAND:
 		return (64);
 	case IFT_FDDI:		/* RFC2467 */