rust: drm: don't pass the address of drm::Device to drm_dev_put()

In drm_dev_put() call in AlwaysRefCounted::dec_ref() we rely on struct
drm_device to be the first field in drm::Device, whereas everywhere
else we correctly obtain the address of the actual struct drm_device.

Analogous to the from_drm_device() helper, provide the
into_drm_device() helper in order to address this.

Fixes: 1e4b889 ("rust: drm: add device abstraction")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250731154919.4132-5-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Danilo Krummrich

rust/kernel/drm/device.rs

@@ -120,9 +120,13 @@ impl<T: drm::Driver> Device<T> {
         // - `raw_data` is a valid pointer to uninitialized memory.
         // - `raw_data` will not move until it is dropped.
         unsafe { data.__pinned_init(raw_data) }.inspect_err(|_| {
-            // SAFETY: `__drm_dev_alloc()` was successful, hence `raw_drm` must be valid and the
+            // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
+            // successful.
+            let drm_dev = unsafe { Self::into_drm_device(raw_drm) };
+
+            // SAFETY: `__drm_dev_alloc()` was successful, hence `drm_dev` must be valid and the
             // refcount must be non-zero.
-            unsafe { bindings::drm_dev_put(ptr::addr_of_mut!((*raw_drm.as_ptr()).dev).cast()) };
+            unsafe { bindings::drm_dev_put(drm_dev) };
         })?;
 
         // SAFETY: The reference count is one, and now we take ownership of that reference as a
@@ -143,6 +147,14 @@ impl<T: drm::Driver> Device<T> {
         unsafe { crate::container_of!(Opaque::cast_from(ptr), Self, dev) }.cast_mut()
     }
 
+    /// # Safety
+    ///
+    /// `ptr` must be a valid pointer to `Self`.
+    unsafe fn into_drm_device(ptr: NonNull<Self>) -> *mut bindings::drm_device {
+        // SAFETY: By the safety requirements of this function, `ptr` is a valid pointer to `Self`.
+        unsafe { &raw mut (*ptr.as_ptr()).dev }.cast()
+    }
+
     /// Not intended to be called externally, except via declare_drm_ioctls!()
     ///
     /// # Safety
@@ -192,8 +204,11 @@ unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {
     }
 
     unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: `obj` is a valid pointer to `Self`.
+        let drm_dev = unsafe { Self::into_drm_device(obj) };
+
         // SAFETY: The safety requirements guarantee that the refcount is non-zero.
-        unsafe { bindings::drm_dev_put(obj.cast().as_ptr()) };
+        unsafe { bindings::drm_dev_put(drm_dev) };
     }
 }