RDMA/mlx5: Get XRCD number directly for the internal use

The mlx5_ib creates XRC domain and uses for creating internal SRQ.
However all that is needed is XRCD number and not full blown ib_xrcd
objects.

Update the code to get and store the number only.

Link: https://lore.kernel.org/r/20200706122716.647338-2-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>