RDMA: Remove constant domain argument from flow creation call

The "domain" argument is constant and modern device (mlx5) doesn't support
anything except IB_FLOW_DOMAIN_USER, so delete this extra parameter and
simplify code.

Link: https://lore.kernel.org/r/20200730081235.1581127-4-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>