RDMA/mlx5: Add missing check for return value in get namespace flow

Add missing check for return value when calling to
mlx5_ib_ft_type_to_namespace, even though it can't really fail in this
specific call.

Fixes: 52438be4

 ("RDMA/mlx5: Allow inserting a steering rule to the FDB")
Link: https://lore.kernel.org/r/7b9ceda217d9368a51dc47a46b769bad4af9ac92.1659256069.git.leonro@nvidia.com
Reviewed-by: Itay Aveksis <itayav@nvidia.com>
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>