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 fc6e66f ("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>

Author: Darksonn

rust/kernel/page.rs

@@ -20,6 +20,16 @@ pub const PAGE_SIZE: usize = bindings::PAGE_SIZE;
 /// A bitmask that gives the page containing a given address.
 pub const PAGE_MASK: usize = !(PAGE_SIZE - 1);
 
+/// Round up the given number to the next multiple of [`PAGE_SIZE`].
+///
+/// It is incorrect to pass an address where the next multiple of [`PAGE_SIZE`] doesn't fit in a
+/// [`usize`].
+pub const fn page_align(addr: usize) -> usize {
+    // Parentheses around `PAGE_SIZE - 1` to avoid triggering overflow sanitizers in the wrong
+    // cases.
+    (addr + (PAGE_SIZE - 1)) & PAGE_MASK
+}
+
 /// A pointer to a page that owns the page allocation.
 ///
 /// # Invariants