IB/mlx4: Don't duplicate the default RoCE GID

When reading the IPv6 addresses from the net-device, make sure to
avoid adding a duplicate entry to the GID table because of equality
between the default GID we generate and the default IPv6 link-local
address of the device.

Fixes: acc4fccf ("IB/mlx4: Make sure GID index 0 is always occupied")
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>