net: ethtool: phy: Don't set the context dev pointer for unfiltered DUMP

The context info allows continuing DUMP requests, shall they fill the
netlink buffer.

In the case of filtered DUMP requests, a reference on the netdev is
grabbed in the .start() callback and release in .done().

Unfiltered DUMP request don't need the dev pointer to be set in the context
info, doing so will trigger an unwanted netdev_put() in .done().

Reported-by: syzbot+e9ed4e4368d450c8f9db@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/000000000000d3bf150621d361a7@google.com/
Fixes: 17194be4 ("net: ethtool: Introduce a command to list PHYs on an interface")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20240913100515.167341-1-maxime.chevallier@bootlin.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ethtool/phy.c

@@ -251,8 +251,6 @@ static int ethnl_phy_dump_one_dev(struct sk_buff *skb, struct net_device *dev,
 	int ret = 0;
 	void *ehdr;
 
-	pri->base.dev = dev;
-
 	if (!dev->link_topo)
 		return 0;