genetlink: move the private fields in struct genl_family

Move the private fields down to form a "private section".
Use the kdoc "private:" label comment thing to hide them
from the main kdoc comment.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>