IB/mlx5: avoid destroying a NULL mr in reg_user_mr error flow

The mlx5_ib_reg_user_mr() function will attempt to call clean_mr() in
its error flow even though there is never a case where the error flow
occurs with a valid MR pointer to destroy.

Remove the clean_mr() call and the incorrect comment above it.

Fixes: b4cfe447 ("IB/mlx5: Implement on demand paging by adding
support for MMU notifiers")
Cc: Eli Cohen <eli@mellanox.com>
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>