mlx5ib: Fix memory leak in clean_mr() error path

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

Linux commit:
5942d8ae411775b76e5e1ab0cce57b0666516f2d

PR: 264653
Sponsored by: NVIDIA Networking

(cherry picked from commit e4d178d093281419c5ad9dea1c497bceb54d0018)