[netdevice] Split multicast hashing out into an mc_hash method

Multicast hashing is an ugly overlap between network and link layers.
EFI requires us to provide access to this functionality, so move it
out of ipv4.c and expose it as a method of the link layer.

src/include/gpxe/netdevice.h

@@ -120,6 +120,16 @@ struct ll_protocol {
 	 * allocated.
 	 */
 	const char * ( * ntoa ) ( const void * ll_addr );
+	/**
+	 * Hash multicast address
+	 *
+	 * @v af	Address family
+	 * @v net_addr	Network-layer address
+	 * @v ll_addr	Link-layer address to fill in
+	 * @ret rc	Return status code
+	 */
+	int ( * mc_hash ) ( unsigned int af, const void *net_addr,
+			    void *ll_addr );
 	/** Link-layer protocol
 	 *
 	 * This is an ARPHRD_XXX constant, in network byte order.