trap: deliver reserved multicast to the CPU by trap, STP first

A reserved-multicast trap goes to the port named in EXT_CPU_CTRL, which
names no port out of reset, so trapped frames were dropped at ingress.
trap_init() points it at the CPU port once at startup, and
trap_rma_set() changes only the action field of an RMA register.

A trapped frame arrives without the VLAN tag a forwarded one gets, so
TRAP_RX_BODY() gives the offset of the payload for both forms.

STP now traps its address while it runs instead of steering it with a
static L2 entry per VLAN, so turning it on no longer walks the VLAN
table and a pvid change no longer needs stp off and on. With STP off
the flooding entries are restored as before.

A trapped BPDU passes the tagged-only ingress filter that drops a
forwarded one, so the stp_setup() warning about such ports goes.

rtl837x_regs.h

@@ -285,6 +285,12 @@
  */
 #define RTL8373_RLDP_TIMER		0x1074
 #define RTL837X_RMA0_CONF		0x4ecc
+#define RTL837X_RMA2_CONF		0x4ed4
+#define RTL837X_RMA_ACT_MASK		0x30
+#define RTL837X_RMA_ACT_FORWARD		0x00
+#define RTL837X_RMA_ACT_TRAP		0x10
+#define RTL837X_RMA_ACT_DROP		0x20
+#define RTL837X_EXT_CPU_CTRL		0x6724
 #define RTL837X_IMR_INT_PORT_LINK_STS_CHG	0x5f34
 #define RTL837X_ISR_SW_INT_MODE		0x5f84
 #define RTL837X_ISR_INT_PORT_LINK_CHG	0x5f88