fou: Fix IPv6 netlink policy

When submitting v2 of "fou: Support binding FoU socket" (1713cb37

),
I accidentally sent the wrong version of the patch and one fix was
missing. In the initial version of the patch, as well as the version 2
that I submitted, I incorrectly used ".type" for the two V6-attributes.
The correct is to use ".len".

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: 1713cb37

 ("fou: Support binding FoU socket")
Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>