net: ethtool: netlink: Use netdev_hold for dumpit() operations

Move away from dev_hold and use netdev_hold with a local reftracker when
performing a DUMP on each netdev.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250502085242.248645-4-maxime.chevallier@bootlin.com


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