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: Daniel Machon <daniel.machon@microchip.com>
Tested-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://lore.kernel.org/r/20240424121632.459022-3-ast@fiberby.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>