tools: ynl: switch away from mnl_cb_t

All YNL parsing callbacks take struct ynl_parse_arg as the argument.
Make that official by using a local callback type instead of mnl_cb_t.

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240227223032.1835527-12-kuba@kernel.org


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