rust: alloc: add Box::into_pin

Add an associated function to convert a `Box<T>` into a `Pin<Box<T>>`.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250309-hrtimer-v3-v6-12-rc2-v12-9-73586e2bd5f1@kernel.org
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Author: Andreas Hindborg

rust/kernel/alloc/kbox.rs

@@ -245,6 +245,12 @@ where
         Ok(Self::new(x, flags)?.into())
     }
 
+    /// Convert a [`Box<T,A>`] to a [`Pin<Box<T,A>>`]. If `T` does not implement
+    /// [`Unpin`], then `x` will be pinned in memory and can't be moved.
+    pub fn into_pin(this: Self) -> Pin<Self> {
+        this.into()
+    }
+
     /// Forgets the contents (does not run the destructor), but keeps the allocation.
     fn forget_contents(this: Self) -> Box<MaybeUninit<T>, A> {
         let ptr = Self::into_raw(this);