RDMA/core: Simplify rdma_addr_get_sgid() to not support RoCE

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: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>