Introduce dumpit callbacks for generated split ops. Have them
as a thin wrapper around iteration function and allow to pass dump_one()
function pointer directly without need to store in devlink_cmd structs.

Note that the function prototypes are temporary until the generated ones
will replace them in a follow-up patch.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230811155714.1736405-6-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>