net/mlx5: DR, Fix peer domain namespace setting

The offending patch is based on the assumption that for PFs,
mlx5_get_dev_index() is the same as vhca_id. However, this assumption
is wrong in case of DPU (ECPF).
Fix it by using vhca_id directly, and switch the array of peers to
xarray.

Fixes: 6d5b7321 ("net/mlx5: DR, handle more than one peer domain")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>