tools: ynl: make sure we always pass yarg to mnl_cb_run

tools: ynl: make sure we always pass yarg to mnl_cb_run

There is one common error handler in ynl - ynl_cb_error().
It expects priv to be a pointer to struct ynl_parse_arg AKA yarg.
To avoid potential crashes if we encounter a stray NLMSG_ERROR
always pass yarg as priv (or a struct which has it as the first
member).

ynl_cb_null() has a similar problem directly - it expects yarg
but priv passed by the caller is ys.

Found by code inspection.

Fixes: 86878f14d71a ("tools: ynl: user space helpers")
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240220161112.2735195-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>