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.com


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