RDMA/mlx5: Add DCS offload support

RDMA/mlx5: Add DCS offload support

DCS is an offload to SW load balancing of DC initiator work requests.

A single DCI can be connected to only one target at the time and can't
start new connection until the previous work request is completed.  This
limitation will cause to delay when the initiator process needs to
transfer data to multiple targets at the same time.  The SW solution is to
use a process that handling and spreading the work request on many DCIs
according to destinations.

This feature is an offload to this process and coming to reduce the load
from the CPU and improve the performance.

Link: https://lore.kernel.org/r/491c2c2afdb5b07de7f03eab3f93cf0704549dbc.1624258894.git.leonro@nvidia.com
Reviewed-by: Meir Lichtinger <meirl@nvidia.com>
Signed-off-by: Lior Nahmanson <liorna@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>