net: stmmac: selftests: Support running selftests on DSA conduits

Most stmmac selftests rely on dev_add_pack() to add custom handlers,
that validate the packets sent to ourselves through MAC loopback.

However, when the stmmac-driven interface is a DSA CPU conduit, all
frames that are received have ETH_P_XDSA as a protocol, even though they
don't actually contain any tag as they come from the loopback and not
the switch.

This will prevent any incoming packet to match our packet handlers.

Let's register a ETH_P_ALL packet handler when we detect that we're a
DSA conduit, and use a proxy packet handler to filter the h_proto.

As this allows external frames to be received through our .func(), the
packet handler is added after the dev->addr field is populated in our
selftest attributes.

Note that we may still receive incoming packets from the switch, but
these frames shouldn't interfere with the very specific frames used for
selftests, and stmmac selftests in general aren't safe against external
traffic interferences.

This was validated on a WPQ864 devkit for IPQ8064, that has the SoC
connected to a QCA8k switch.

The ARP offload's packet handler is left alone, this feature is just not
implemented in stmmac and due for removal.

Fixes: 091810dbded9 ("net: stmmac: Introduce selftests support")
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260917215339.2022523-2-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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