rust: alloc: layout: implement ArrayLayout::size()

Provide a convenience method for ArrayLayout to calculate the size of
the ArrayLayout in bytes.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Link: https://lore.kernel.org/r/20250820145434.94745-7-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Danilo Krummrich

rust/kernel/alloc/layout.rs

@@ -98,6 +98,11 @@ impl<T> ArrayLayout<T> {
     pub const fn is_empty(&self) -> bool {
         self.len == 0
     }
+
+    /// Returns the size of the [`ArrayLayout`] in bytes.
+    pub const fn size(&self) -> usize {
+        self.len() * core::mem::size_of::<T>()
+    }
 }
 
 impl<T> From<ArrayLayout<T>> for Layout {