Add missing netlink attribute policy and size calculation.
Also enable strict validation from this new attribute onwards.

Fixes: 435fe1c0 ("net: geneve: support IPv4/IPv6 as inner protocol")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Link: https://lore.kernel.org/r/20220322043954.3042468-1-eyal.birger@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>