rust: alloc: Fix `ArrayLayout` allocations

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: 9e7bbfa18276 ("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]>