IB/mlx5: Fix memory leak in clean_mr error path

In clean_mr error path the 'mr' should be freed.

Fixes: e126ba97

 ('mlx5: Add driver for Mellanox Connect-IB adapters')
Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>