Fix Ethernet addresses in loopback packets

Fixes a bug where the net tile uses MAC addresses of real interfaces
when sending packets over the loopback XSK.  This is only a cosmetic
fix.  This bug does not seem to have any effect.

src/app/fdctl/run/tiles/fd_net.c

@@ -415,6 +415,9 @@ after_frag( fd_net_ctx_t *      ctx,
 
   fd_aio_pkt_info_t aio_buf = { .buf = ctx->frame, .buf_sz = (ushort)sz };
   if( FD_UNLIKELY( route_loopback( ctx->src_ip_addr, sig ) ) ) {
+    /* Set Ethernet src and dst address to 00:00:00:00:00:00 */
+    memset( ctx->frame, 0, 12UL );
+
     ulong sent_cnt;
     int aio_err = ctx->lo_tx->send_func( ctx->xsk_aio[ 1 ], &aio_buf, 1, &sent_cnt, 1 );
     ctx->metrics.tx_dropped_cnt += aio_err!=FD_AIO_SUCCESS;