net: dsa: realtek: fix out-of-bounds access

The probe function sets priv->chip_data to (void *)priv + sizeof(*priv)
with the expectation that priv has enough trailing space.

However, only realtek-smi actually allocated this chip_data space.
Do likewise in realtek-mdio to fix out-of-bounds accesses.

These accesses likely went unnoticed so far, because of an (unused)
buf[4096] member in struct realtek_priv, which caused kmalloc to
round up the allocated buffer to a big enough size, so nothing of
value was overwritten. With a different allocator (like in the barebox
bootloader port of the driver) or with KASAN, the memory corruption
becomes quickly apparent.

Fixes: aac94001067d ("net: dsa: realtek: add new mdio interface for drivers")
Reviewed-by: Florian Fainelli <[email protected]>
Reviewed-by: Luiz Angelo Daros de Luca <[email protected]>
Reviewed-by: Alvin Šipraga <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Signed-off-by: Ahmad Fatoum <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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