net: mdio: fix unbalanced fwnode reference count in mdio_device_release()

In the Linux kernel, the following vulnerability has been resolved:

net: mdio: fix unbalanced fwnode reference count in mdio_device_release()

There is warning report about of_node refcount leak

while probing mdio device:

OF: ERROR: memory leak, expected refcount 1 instead of 2,

of_node_get()/of_node_put() unbalanced - destroy cset entry:

attach overlay node /spi/soc@0/mdio@710700c0/ethernet@4

In of_mdiobus_register_device(), we increase fwnode refcount

by fwnode_handle_get() before associating the of_node with

mdio device, but it has never been decreased in normal path.

Since that, in mdio_device_release(), it needs to call

fwnode_handle_put() in addition instead of calling kfree()

directly.

After above, just calling mdio_device_free() in the error handle

path of of_mdiobus_register_device() is enough to keep the

refcount balanced.