realtek: l3: rtl839x: trap packets to an aged-out gateway

On RTL839x a route forwards through its gateway's L2 entry. A gateway
that sends nothing lets that entry age out, and a packet routed
through it then hits the routing entry age-out exception, whose action
is forward after reset: the packet is routed and also reaches the CPU,
which routes it a second time when IPv4 forwarding is on.

Trap it instead (ROUTING_AGE, bits 15:14 of ROUTING_EXCPT_CTRL: 0 drop,
1 forward, 2 trap; Realtek GPL SDK, dal_cypress_trap.c, exception
ROUTE_EXCEPTION_TYPE_ENTRY_AGE_OUT), as the TTL exception already does.
The CPU routes such packets alone until the gateway sends again and
its entry is learnt anew.

Measured on an HPE 1920-48G (RTL8393), 10.99.0.0/16 via a gateway
that sends nothing, 8 echo requests per case, once the age of its L2
entry had run down to 0 (about 4.5 minutes on the bench):
- without this change: forwarding off, 8/8 routed and 8/8 at the CPU
  as well; forwarding on, 14 at the gateway, 8 from the switch and 6
  from the kernel
- with it: forwarding off, none routed, 8/8 at the CPU; forwarding
  on, 8 at the gateway, none of them twice
With a fresh entry, 8/8 were routed in hardware and none reached the
CPU, without and with this change. Both images also carried the
pending switch MAC change for #25768 items 3 and 4 and bench debugfs
files.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Gennaro Cimmino <gcimmino@rayonra.net>
Link: https://github.com/openwrt/openwrt/pull/25821
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

target/linux/realtek/files-6.18/drivers/net/dsa/rtl83xx/l3.c

@@ -296,6 +296,13 @@ static int otto_l3_839x_setup(struct otto_l3_ctrl *ctrl)
 	 */
 	sw_w32_mask(0x3 << 2, 2 << 2, RTL839X_ROUTING_EXCPT_CTRL);
 
+	/* A packet routed through a gateway whose L2 entry has aged out goes
+	 * to the CPU alone. Forwarded, the default, it is routed and reaches
+	 * the CPU as well, which routes it a second time. ROUTING_AGE, bits
+	 * 15:14, same encoding.
+	 */
+	sw_w32_mask(0x3 << 14, 2 << 14, RTL839X_ROUTING_EXCPT_CTRL);
+
 	return 0;
 }