ipv4: use tunnel flow flags for tunnel route lookups

Commit 451ef36b ("ip_tunnels: Add new flow flags field to
ip_tunnel_key") added a new field to struct ip_tunnel_key to control
route lookups. Currently the flag is used by vxlan and geneve tunnels;
use it also in udp_tunnel_dst_lookup() so that it affects all tunnel
types relying on this function.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>