net/mlx5: Avoid unnecessary operation

fs_get_obj retrieves the container of fs_parent_node just to pass the
same value as &fs_ns->node. Just pass fs_parent_node to
init_root_tree_recursive() to get exactly the same effect.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>