net: phy: clear phydev->devlink when the link is deleted

net: phy: clear phydev->devlink when the link is deleted

There is a potential crash issue when disabling and re-enabling the
network port. When disabling the network port, phy_detach() calls
device_link_del() to remove the device link, but it does not clear
phydev->devlink, so phydev->devlink is not a NULL pointer. Then the
network port is re-enabled, but if phy_attach_direct() fails before
calling device_link_add(), the code jumps to the "error" label and
calls phy_detach(). Since phydev->devlink retains the old value from
the previous attach/detach cycle, device_link_del() uses the old value,
which accesses a NULL pointer and causes a crash. The simplified crash
log is as follows.

[   24.702421] Call trace:
[   24.704856]  device_link_put_kref+0x20/0x120
[   24.709124]  device_link_del+0x30/0x48
[   24.712864]  phy_detach+0x24/0x168
[   24.716261]  phy_attach_direct+0x168/0x3a4
[   24.720352]  phylink_fwnode_phy_connect+0xc8/0x14c
[   24.725140]  phylink_of_phy_connect+0x1c/0x34

Therefore, phydev->devlink needs to be cleared when the device link is
deleted.

Fixes: bc66fa87d4fd ("net: phy: Add link between phy dev and mac dev")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20250523083759.3741168-1-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>