net/rds: Use rdma_read_gids to read connection GIDs

Use the newly introduced rdma_read_gids() to read the SGID and DGID for
the connection which returns GID correctly for RoCE transport as well.

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.

Therefore use transport agnostic rdma_read_gids() API provided by rdma_cm
module.

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