selftests: mlxsw: extack: Disable IPv6 autogen on bridges

In a future patch, mlxsw will start adding RIFs to uppers of front panel
port netdevices, if they have an IP address.

At the time that the front panel port is enslaved to the bridge (this holds
for all bridges used here), the bridge MAC address does not have the same
prefix as other interfaces in the system. On Nvidia Spectrum-1 machines all
the RIFs have to have the same 38-bit MAC address prefix. Since the bridge
does not obey this limitation, the RIF cannot be created, and the
enslavement attempt is vetoed on the grounds of the configuration not being
offloadable.

The selftest itself however checks whether a different vetoed aspect of the
configuration provides an extack. The IP address or the RIF are irrelevant.

Fix by disabling automatic IPv6 address generation for the HW-offloaded
bridges in this selftest, thus exempting them from mlxsw router attention.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>