netlink: specs: fou: change local-v6/peer-v6 check

netlink: specs: fou: change local-v6/peer-v6 check

While updating the binary min-len implementation, I noticed that
the only user, should AFAICT be using exact-len instead.

In net/ipv4/fou_core.c FOU_ATTR_LOCAL_V6 and FOU_ATTR_PEER_V6
are only used for singular IPv6 addresses, and there are AFAICT
no known implementations trying to send more, it therefore
appears safe to change it to an exact-len policy.

This patch therefore changes the local-v6/peer-v6 attributes to
use an exact-len check, instead of a min-len check.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250902154640.759815-2-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>