rust: page: add Rust version of PAGE_ALIGN

This is a useful for helper for working with indices into buffers that
consist of several pages. I forgot to include it when I added PAGE_SIZE
and PAGE_MASK for the same purpose in commit fc6e66f4 ("rust: add
abstraction for `struct page`").

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241016-page-align-v2-1-e0afe85fc4b4@google.com


[ Added intra-doc links, formatted comment and replaced "Brackets" with
  "Parentheses" as discussed in the list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>