selftests: forwarding: mirror_gre_*: 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, 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.

These two selftests however check mirroring traffic to a gretap netdevice.
The bridge here does not participate in routing traffic and the IP address
or the RIF are irrelevant.

Fix by disabling automatic IPv6 address generation for the HW-offloaded
bridges in these selftests, thus exempting them from mlxsw router
attention. Since the bridges are only used for L2 forwarding, this change
should not hinder usefulness of this selftest for testing SW datapath or HW
datapaths in other devices.

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