tools: ynl: use MSG_DONTWAIT for getting notifications

To stick to libmnl wrappers in the past we had to use poll()
to check if there are any outstanding notifications on the socket.
This is no longer necessary, we can use MSG_DONTWAIT.

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240227223032.1835527-16-kuba@kernel.org


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