genetlink: push conditional locking into dumpit/done

Add helpers which take/release the genl mutex based
on family->parallel_ops. Remove the separation between
handling of ops in locked and parallel families.

Future patches would make the duplicated code grow even more.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-2-kuba@kernel.org


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