rust: Return Option from page_align and ensure no usize overflow

rust: Return Option from page_align and ensure no usize overflow

Change `page_align()` to return `Option<usize>` to allow validation
of the provided `addr` value. This ensures that any value that is
within one `PAGE_SIZE` of `usize::MAX` will not panic, and instead
returns `None` to indicate overflow.

Signed-off-by: Brendan Shephard <bshephar@bne-home.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://patch.msgid.link/20251223055647.9761-1-bshephar@bne-home.net
[ Use kernel vertical style for imports; use markdown in comments.
  - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>