IB/mlx4: Make sure GID index 0 is always occupied

Make sure that for Ethernet ports, the port GID table index 0 is always
occupied with a default GID of the relevant IPv6 link-local adderss.

This provides better user experience for legacy applications that don't use
the RDMA CM and were working on index 0 prior to the IP addressing change.

Also, as GIDs are generated from IP addresses of the network devices that
are associated with the port, it's basically possible that the GID table
will be empty if no IP address was assigned.  This doesn't comply with the
IB spec section 4.1.1 "GID usage and properties".

Signed-off-by: Moni Shoua <monis@mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>