Add support for IPv6 link local GIDs equal to the default GID for

Add support for IPv6 link local GIDs equal to the default GID for
VLANs in ibcore.

IPv6 link local addresses are usually derived from the netdev MAC
address. This is applicable to VLAN devices and its lower netdevice as
well. In such cases the IPv6 link local address is a duplicate of the
default GID.

Now that link local IPv6 addresses based GIDs are supported, allow
adding such GID entries in the GID table.

MFC after:	1 week
Sponsored by:	Mellanox Technologies