openvswitch: warn about missing first netlink attribute

The first netlink attribute (value 0) must always be defined
as none/unspec.

Because we cannot change an existing UAPI, I add a comment to point the
mistake and avoid to propagate it in a new ovs API in the future.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>