vxlan: use generic function for tunnel IPv6 route lookup

The route lookup can be done now via generic function
udp_tunnel6_dst_lookup() to replace the custom implementation in
vxlan6_get_route().

This is similar to what already done for IPv4 in commit 6f19b2c1
("vxlan: use generic function for tunnel IPv4 route lookup").

Suggested-by: Guillaume Nault <gnault@redhat.com>
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>