IB/mlx4: Fix use-after-free on pkey sysfs registration failure

In the Linux kernel, the following vulnerability has been resolved: IB/mlx4: Fix use-after-free on pkey sysfs registration failure register_pkey_tree() ignores errors from register_one_pkey_tree() and continues registering the remaining slaves. The per-slave error path has already released the pkey parent kobjects, but their pointers remain stored in the device. A later device cleanup therefore passes the stale pointers to kobject_put(), causing a use-after-free. Clear the parent pointers after releasing a failed slave tree and skip unregistered trees during device cleanup. This preserves the existing best-effort registration behavior while preventing a second cleanup of the failed tree.