rust: alloc: replace aligned_size() with Kmalloc::aligned_layout()

rust: alloc: replace aligned_size() with Kmalloc::aligned_layout()

aligned_size() dates back to when Rust did support kmalloc() only, but
is now used in ReallocFunc::call() and hence for all allocators.

However, the additional padding applied by aligned_size() is only
required by the kmalloc() allocator backend.

Hence, replace aligned_size() with Kmalloc::aligned_layout() and use it
for the affected allocators, i.e. kmalloc() and kvmalloc(), only.

While at it, make Kmalloc::aligned_layout() public, such that Rust
abstractions, which have to call subsystem specific kmalloc() based
allocation primitives directly, can make use of it.

Fixes: 8a799831fc63 ("rust: alloc: implement `ReallocFunc`")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250731154919.4132-2-dakr@kernel.org
[ Remove `const` from Kmalloc::aligned_layout(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>