net/ipv6/udp_tunnel: prefer SO_BINDTOIFINDEX over SO_BINDTODEVICE

The udp-tunnel setup allows binding sockets to a network device. Prefer
the new SO_BINDTOIFINDEX to avoid temporarily resolving the device-name
just to look it up in the ioctl again.

Reviewed-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>