sample: bpf: xdp_router_ipv4: Allow the kernel to send arp requests

Forward the packet to the kernel if the gw router mac address is missing
in to trigger ARP discovery.

Fixes: 85bf1f5169 ("samples: bpf: Convert xdp_router_ipv4 to XDP samples helper")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/60bde5496d108089080504f58199bcf1143ea938.1653471558.git.lorenzo@kernel.org

samples/bpf/xdp_router_ipv4.bpf.c

@@ -150,6 +150,15 @@ int xdp_router_ipv4_prog(struct xdp_md *ctx)
 
 				dest_mac = bpf_map_lookup_elem(&arp_table,
 							       &prefix_value->gw);
+				if (!dest_mac) {
+					/* Forward the packet to the kernel in
+					 * order to trigger ARP discovery for
+					 * the default gw.
+					 */
+					if (rec)
+						NO_TEAR_INC(rec->xdp_pass);
+					return XDP_PASS;
+				}
 			}
 		}