net/mlx5: E-Switch, Initialize MAC Address for Default GID

Initialize the source MAC address when creating the default GID entry.
Since this entry is used only for loopback traffic, it only needs to
be a unicast address. A zeroed-out MAC address is sufficient for this
purpose.
Without this fix, random bits would be assigned as the source address.
If these bits formed a multicast address, the firmware would return an
error, preventing the user from switching to switchdev mode:

Error: mlx5_core: Failed setting eswitch to offloads.
kernel answers: Invalid argument

Fixes: 80f09dfc ("net/mlx5: Eswitch, enable RoCE loopback traffic")
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Link: https://patch.msgid.link/20250423083611.324567-3-mbloch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>