selftests/net: Fix tun IPv6 test addresses to avoid 6to4 range

selftests/net: Fix tun IPv6 test addresses to avoid 6to4 range

The IPv6 addresses used for the tun_vnet_udptnl fixture currently fall in
the 2002::/16 prefix, which is reserved for the 6to4 transition mechanism
(RFC 3056).

On systems where the sit module is loaded, the kernel automatically claims
2002::/16 as a 6to4 tunnel prefix. When the test assigns a 2002:: address
to a TUN interface, sit registers a competing local route for the same
address. This ambiguity breaks the GENEVE decapsulation path: packets
injected via the TUN fd are not delivered to the test socket, causing the
IPv6-outer gtgso send_gso_packet variants to fail.

Replace all four IPv6 test addresses with addresses from the fd00:db8::/32
range, which is part of the ULA space (fc00::/7, RFC 4193) and carries no
special kernel semantics.

Fixes: 24e59f26eef2 ("selftest: tun: Add helpers for GSO over UDP tunnel")
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260706-b4-net_tun_addr-v1-1-3d3cb2473560@suse.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>