tools: ynl: stop using mnl_cb_run2()

There's only one set of callbacks in YNL, for netlink control
messages, and most of them are trivial. So implement the message
walking directly without depending on mnl_cb_run2().

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240227223032.1835527-11-kuba@kernel.org


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