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: 85bf1f51 ("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