rust: drm: gem: shmem: Fix Default implementation for ObjectConfig

I completely forgot when coming up with this type that #[derive(Default)]
only works if all generics mentioned in the type implement Default (and T
usually doesn't). This being said: We don't use `T` for anything besides
using it for a reference type, so whether or not it implements `Default`
shouldn't actually need to matter.

So, fix this by just manually implementing Default instead of deriving it.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260603195210.693856-2-lyude@redhat.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

rust/kernel/drm/gem/shmem.rs

@@ -42,7 +42,6 @@
 ///
 /// This is used with [`Object::new()`] to control various properties that can only be set when
 /// initially creating a shmem-backed GEM object.
-#[derive(Default)]
 pub struct ObjectConfig<'a, T: DriverObject, C: DeviceContext = Registered> {
     /// Whether to set the write-combine map flag.
     pub map_wc: bool,
@@ -53,6 +52,16 @@ pub struct ObjectConfig<'a, T: DriverObject, C: DeviceContext = Registered> {
     pub parent_resv_obj: Option<&'a Object<T, C>>,
 }
 
+impl<'a, T: DriverObject, C: DeviceContext> Default for ObjectConfig<'a, T, C> {
+    #[inline(always)]
+    fn default() -> Self {
+        Self {
+            map_wc: false,
+            parent_resv_obj: None,
+        }
+    }
+}
+
 /// A shmem-backed GEM object.
 ///
 /// # Invariants