[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/drivers/net/ipoib.c

 	return buf;
 }
 
+/**
+ * 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
+ */
+static int ipoib_mc_hash ( unsigned int af __unused,
+			   const void *net_addr __unused,
+			   void *ll_addr __unused ) {
+
+	return -ENOTSUP;
+}
+
 /** IPoIB protocol */
 struct ll_protocol ipoib_protocol __ll_protocol = {
 	.name		= "IPoIB",
 	.push		= ipoib_push,
 	.pull		= ipoib_pull,
 	.ntoa		= ipoib_ntoa,
+	.mc_hash	= ipoib_mc_hash,
 };
 
 /****************************************************************************

src/include/gpxe/netdevice.h

 	 * 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.

src/net/ethernet.c

 #include <assert.h>
 #include <gpxe/if_arp.h>
 #include <gpxe/if_ether.h>
+#include <gpxe/in.h>
 #include <gpxe/netdevice.h>
 #include <gpxe/iobuf.h>
 #include <gpxe/ethernet.h>
 /**
  * Transcribe Ethernet address
  *
- * @v ll_addr	Link-layer address
- * @ret string	Link-layer address in human-readable format
+ * @v ll_addr		Link-layer address
+ * @ret string		Link-layer address in human-readable format
  */
 const char * eth_ntoa ( const void *ll_addr ) {
 	static char buf[18]; /* "00:00:00:00:00:00" */
 	return buf;
 }
 
+/**
+ * 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
+ */
+static int eth_mc_hash ( unsigned int af, const void *net_addr,
+			 void *ll_addr ) {
+	const uint8_t *net_addr_bytes = net_addr;
+	uint8_t *ll_addr_bytes = ll_addr;
+
+	switch ( af ) {
+	case AF_INET:
+		ll_addr_bytes[0] = 0x01;
+		ll_addr_bytes[1] = 0x00;
+		ll_addr_bytes[2] = 0x5e;
+		ll_addr_bytes[3] = net_addr_bytes[1] & 0x7f;
+		ll_addr_bytes[4] = net_addr_bytes[2];
+		ll_addr_bytes[5] = net_addr_bytes[3];
+	default:
+		return -ENOTSUP;
+	}
+}
+
 /** Ethernet protocol */
 struct ll_protocol ethernet_protocol __ll_protocol = {
 	.name		= "Ethernet",
 	.push		= eth_push,
 	.pull		= eth_pull,
 	.ntoa		= eth_ntoa,
+	.mc_hash	= eth_mc_hash,
 };

src/net/ipv4.c

 static int ipv4_ll_addr ( struct in_addr dest, struct in_addr src,
 			  struct net_device *netdev, uint8_t *ll_dest ) {
 	struct ll_protocol *ll_protocol = netdev->ll_protocol;
-	uint8_t *dest_bytes = ( ( uint8_t * ) &dest );
 
 	if ( dest.s_addr == INADDR_BROADCAST ) {
 		/* Broadcast address */
 			 ll_protocol->ll_addr_len );
 		return 0;
 	} else if ( IN_MULTICAST ( ntohl ( dest.s_addr ) ) ) {
-		/* Special case: IPv4 multicast over Ethernet.	This
-		 * code may need to be generalised once we find out
-		 * what happens for other link layers.
-		 */
-		ll_dest[0] = 0x01;
-		ll_dest[1] = 0x00;
-		ll_dest[2] = 0x5e;
-		ll_dest[3] = dest_bytes[1] & 0x7f;
-		ll_dest[4] = dest_bytes[2];
-		ll_dest[5] = dest_bytes[3];
-		return 0;
+		return ll_protocol->mc_hash ( AF_INET, &dest, ll_dest );
 	} else {
 		/* Unicast address: resolve via ARP */
 		return arp_resolve ( netdev, &ipv4_protocol, &dest,