IB/mlx5: Fix clean_mr() to work in the expected order

Any dma map underlying the MR should only be freed once the MR is fenced
at the hardware.

As of the above we first destroy the MKEY and just after that can safely
call to dma_unmap_single().

Link: https://lore.kernel.org/r/20190723065733.4899-6-leon@kernel.org
Cc: <stable@vger.kernel.org> # 4.3
Fixes: 8a187ee5

 ("IB/mlx5: Support the new memory registration API")
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>