ipip: specify af for a tunnel ip addresses

ipip: specify af for a tunnel ip addresses

Currently ipip tunnel allows to use an empty addresses for dst when we
trying to add tunnel, but it doesn't return it correctly on ipip dump
call. There is case when we create an ipip tunnel with an empty ipv6
address, address is zero, but AF is ipv6. And when we dump ipip tunnels
we get an emtpy ipv4 address instead of an empty ipv6 address.
There is no point to detect AF for each address in
send_ipip_tunnel_details, because ipip tunnel can't handle different
AF for src and dst addresses, and prohibits creating ipip tunnel
with different AF of dst and src addresses.

With this fix, send_ipip_tunnel_details function return a correct AF
despite address value.

Type: fix
Change-Id: Ib343293ad79a300cdb70135fffbfd156dfef6e27
Signed-off-by: Anton Nikolaev <anikolaev@netgate.com>