netdev: support dumping a single netdev in qstats

Having to filter the right ifindex in the tests is a bit tedious.
Add support for dumping qstats for a single ifindex.

Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240420023543.3300306-2-kuba@kernel.org


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