MISSING SIGNOFFS: rust: Add PAGE_SIZE constant to kernel crate

Computed based on the PAGE_SHIFT macro from C.

// Co-authored-by: Adam Bratschi-Kaye <ark.email@gmail.com>
// Co-authored-by: Miguel Ojeda <ojeda@kernel.org>
// Signed-off-by: Asahi Lina <lina@asahilina.net>

Author: hoshinolina

rust/kernel/lib.rs

@@ -99,6 +99,11 @@ pub(crate) mod private {
     pub trait Sealed {}
 }
 
+/// Page size defined in terms of the `PAGE_SHIFT` macro from C.
+///
+/// [`PAGE_SHIFT`]: ../../../include/asm-generic/page.h
+pub const PAGE_SIZE: usize = 1 << bindings::PAGE_SHIFT;
+
 /// Prefix to appear before log messages printed from within the `kernel` crate.
 const __LOG_PREFIX: &[u8] = b"rust_kernel\0";