netlink: specs: add dump-strict flag for dont-validate property

Allow user to specify GENL_DONT_VALIDATE_DUMP_STRICT flag for validation
and add this flag to netlink spec schema.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230803111340.1074067-2-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>