rust: allocator: add KUnit tests for alignment guarantees

rust: allocator: add KUnit tests for alignment guarantees

Add a test module to verify memory alignment guarantees for Rust kernel
allocators.  The tests cover `Kmalloc`, `Vmalloc` and `KVmalloc`
allocators with both standard and large page-aligned allocations.

Key features of the tests:
1. Creates alignment-constrained types:
   - 128-byte aligned `Blob`
   - 8192-byte (4-page) aligned `LargeAlignBlob`
2. Validates allocators using `TestAlign` helper which:
   - Checks address alignment masks
   - Supports uninitialized allocations
3. Tests all three allocators with both alignment requirements:
   - Kmalloc with 128B and 8192B
   - Vmalloc with 128B and 8192B
   - KVmalloc with 128B and 8192B

Link: https://lkml.kernel.org/r/d2e3d6454c1435713be0fe3c0dc444d2c60bba51.1753929369.git.zhuhui@kylinos.cn
Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Reviewed-by: Kunwu Chan <chentao@kylinos.cn>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Trevor Gross <tmgross@umich.edu>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>