geneve: Use ip_tunnel_key flow flags in route lookups

Use the new ip_tunnel_key field with the flow flags in the IPv4 route
lookups for the encapsulated packet. This will be used by the
bpf_skb_set_tunnel_key helper in the subsequent commit.

Signed-off-by: Paul Chaignon <paul@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/fcc2e0eea01e8ea465a180126366ec20596ba530.1658759380.git.paul@isovalent.com