realtek: dsa: rtl839x: add a port to its own isolation mask

The RTL839x applies a port's isolation mask to every frame that came in
on it, routed ones included. The driver leaves the port out of its own
mask, so a route offloaded through a gateway behind the port a packet
came in on, such as routing between two VLANs of one trunk, drops
everything: the kernel shows the route as offloaded, and the drop
counter PORT_ISOLATION counts each packet.

Keep the port in its own mask, as the GPL SDK does at init
(_dal_cypress_port_init_config() sets every port's mask to all ports).
A broadcast is still not sent back out of the port it came in on. Only
RTL839x changes; the other families keep their masks as they are.

Tested on an HPE 1920-48G (RTL8393), with 10.99.1.0/24 routed from VLAN
7 to a gateway on VLAN 3, both behind lan48. On main, 0/8 echo requests
reached the gateway, even with IPv4 forwarding on in the kernel. With
this change and forwarding off, so that only the hardware path delivers,
8/8 arrived, TTL 63, from the switch MAC. Routing to a gateway behind
lan47 still delivers 8/8. Two bursts of 50 broadcasts entering lan48
added nothing to its tx_BroadcastPkts, while the same entering lan47
added 50 and 53.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Gennaro Cimmino <gcimmino@rayonra.net>
Link: https://github.com/openwrt/openwrt/pull/25764
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

target/linux/realtek/files-6.18/drivers/net/dsa/rtl83xx/rtl839x.c

@@ -97,7 +97,12 @@ static inline int rtl839x_mac_port_ctrl(int p)
 
 static void rtl839x_traffic_set(int source, u64 dest_matrix)
 {
-	rtl839x_set_port_reg_be(dest_matrix, rtl839x_port_iso_ctrl(source));
+	/* The isolation mask also applies to frames routed in hardware, which
+	 * may have to leave through the port they came in on. The SDK keeps a
+	 * port in its own mask; a bridged frame is still not sent back out of
+	 * its ingress port.
+	 */
+	rtl839x_set_port_reg_be(dest_matrix | BIT_ULL(source), rtl839x_port_iso_ctrl(source));
 }
 
 static void rtl839x_traffic_enable(int source, int dest)