Now that all callers who care about RoCE addresses have been
converted to use rdma_read_gids() simplify rdma_addr_get_sgid()
to only support real GID addresses.

Callers should only use it for OPA and IB transports.

The now deleted implementation for RoCE has several bugs related to IPv6
support and incorrect/inconsistent 'GID' addresses compared to the CM
paths.
Signed-off-by: NParav Pandit <parav@mellanox.com>
Reviewed-by: NDaniel Jurgens <danielj@mellanox.com>
Signed-off-by: NLeon Romanovsky <leon@kernel.org>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>