netdev-genl: avoid empty messages in queue dump

Empty netlink responses from do() are not correct (as opposed to
dump() where not dumping anything is perfectly fine).
We should return an error if the target object does not exist,
in this case if the netdev is down it has no queues.

Fixes: 6b6171db ("netdev-genl: Add netlink framework functions for queue")
Reported-by:  <syzbot+0a884bc2d304ce4af70f@syzkaller.appspotmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20241218022508.815344-1-kuba@kernel.org


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