IB/mlx5: Clean mr_cache debugfs in case of failure

The failure in creation of debugfs entries for mr_cache left entries,
which were already created.

It caused to mismatch and misguiding for the end users. The solution
is to clean mr_cache debugfs root, so no leftovers will be in the
system. In addition, let's document why the error is not needed to be
forwarded to user in case of failure.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>