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

[ Upstream commit b93eb564869321d0dffaf23fcc5c88112ed62466 ]

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 <f.fainelli@gmail.com>
Reviewed-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.kernel.org/r/20230323103735.2331786-1-a.fatoum@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

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