net: geneve: add missing netlink policy and size for IFLA_GENEVE_INNER_PROTO_INHERIT

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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>