rust: drm: device: drop_in_place() the drm::Device in release()

In drm::Device::new() we allocate with __drm_dev_alloc() and return an
ARef<drm::Device>.

When the reference count of the drm::Device falls to zero, the C code
automatically calls drm_dev_release(), which eventually frees the memory
allocated in drm::Device::new().

However, due to that, drm::Device::drop() is never called. As a result
the destructor of the user's private data, i.e. drm::Device::data is
never called. Hence, fix this by calling drop_in_place() from the DRM
device's release callback.

Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
Reviewed-by: Alice Ryhl <[e-mail]>
Signed-off-by: Danilo Krummrich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

rust/kernel/drm/device.rs

···
         open: Some(drm::File::<T::File>::open_callback),
         postclose: Some(drm::File::<T::File>::postclose_callback),
         unload: None,
-        release: None,
+        release: Some(Self::release),
         master_set: None,
         master_drop: None,
         debugfs_init: None,
···
 
         // SAFETY: `ptr` is valid by the safety requirements of this function.
         unsafe { &*ptr.cast() }
+    }
+
+    extern "C" fn release(ptr: *mut bindings::drm_device) {
+        // SAFETY: `ptr` is a valid pointer to a `struct drm_device` and embedded in `Self`.
+        let this = unsafe { Self::from_drm_device(ptr) };
+
+        // SAFETY:
+        // - When `release` runs it is guaranteed that there is no further access to `this`.
+        // - `this` is valid for dropping.
+        unsafe { core::ptr::drop_in_place(this) };
     }
 }