netlink: move extack writing helpers

Next change will need them in netlink_dump_done(), pure move.

Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240420023543.3300306-3-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>