IB/mlx5: Add DCT RoCE LAG support

When DCT QPs work in RoCE LAG mode:
 1. DCT creation is allowed only when it is supported
 2. The "port" of a DCT QP is assigned in a round-robin way

Link: https://lore.kernel.org/r/20200818115245.700581-3-leon@kernel.org


Signed-off-by: Mark Zhang <markz@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>