net/rds: Use rdma_read_gids to get connection SGID/DGID in IPv6

In IPv4, the newly introduced rdma_read_gids is used to read the SGID/DGID
for the connection which returns GID correctly for RoCE transport as well.

In IPv6, rdma_read_gids is also used. The following are why rdma_read_gids
is introduced.

rdma_addr_get_dgid() for RoCE for client side connections returns MAC
address, instead of DGID.
rdma_addr_get_sgid() for RoCE doesn't return correct SGID for IPv6 and
when more than one IP address is assigned to the netdevice.

So the transport agnostic rdma_read_gids() API is provided by rdma_cm
module.

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>