tools: ynl: create local for_each helpers

Create ynl_attr_for_each*() iteration helpers.
Use them instead of the mnl ones.

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240227223032.1835527-4-kuba@kernel.org


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