selftests: drv-net: Introduce a selftest for ethtool flow control

Ethernet flow control is a tricky thing to get right, especially when it
comes to correctly handling the negotiation of the parameters with a
link partner.

On one hand, the userspace API talks in terms of the device's ability to
send Pause frames when the device is overwhelmed by ingress traffic (TX
pause) and the ability to stop sending traffic upon receiving Pause
frames (RX pause).

On the other hand, 802.3 explains that devices can exchange their
abilities over link negotiation, but instead of echanging TX and RX
abilities, they exchange Pause and Asymmetric Pause capabilities :

RX unable TX unable => None
RX able   TX unable => Pause + Asymmetric
RX unable TX able   => Asymmetric
RX able   TX able   => Pause

The reported Pause and Asym abilities depend both on the MAC, that
eventually sends and processes these frames, and the PHY, that
advertises these to the partner.

Both MAC and PHYs can have their own limitations, and getting the
correct set of supported Pause/Asym parameters is non-trivial, unless
the MAC uses phylink, which deals with the complexity.

Introduce a set of Pause selftests that verify that the local
interface's supported pause parameters reported from ethtool (in terms
of Pause + Asym ) match the accepted parameters from "ethtool -A",
corresponding to ethtool's .set_pauseparams() ops, expressed in TX and
RX abilities.

Signed-off-by: Maxime Chevallier (Netdev Foundation) <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20261015134350.629593-2-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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