Define extack locally, to reduce line lengths and aid future users.

Only compile-tested.
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20240424125347.461995-2-ast@fiberby.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>