rust: alloc: Fix ArrayLayout allocations

We were accidentally allocating a layout for the *square* of the object
size due to a variable shadowing mishap.

Fixes memory bloat and page allocation failures in drm/asahi.

Reported-by: Janne Grunau <[email protected]>
Fixes: 9e7bbfa ("rust: alloc: introduce `ArrayLayout`")
Signed-off-by: Asahi Lina <[email protected]>
Acked-by: Danilo Krummrich <[email protected]>
Reviewed-by: Neal Gompa <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

rust/kernel/alloc/layout.rs

@@ -45,7 +45,7 @@ impl<T> ArrayLayout<T> {
     /// When `len * size_of::<T>()` overflows or when `len * size_of::<T>() > isize::MAX`.
     pub const fn new(len: usize) -> Result<Self, LayoutError> {
         match len.checked_mul(core::mem::size_of::<T>()) {
-            Some(len) if len <= ISIZE_MAX => {
+            Some(size) if size <= ISIZE_MAX => {
                 // INVARIANT: We checked above that `len * size_of::<T>() <= isize::MAX`.
                 Ok(Self {
                     len,