geneve: use generic function for tunnel IPv4 route lookup

The route lookup can be done now via generic function
udp_tunnel_dst_lookup() to replace the custom implementation in
geneve_get_v4_rt().

Suggested-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>