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

[ Upstream commit 5d1a04f347e6cbf5ffe74da409a5d71fbe8c5f19 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>