Add offset_in_page to KernelSpace

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I095e626621f383acdc269db5cd3ae0b68e9285ec
diff --git a/src/kernel_space.rs b/src/kernel_space.rs
index 66a65c1..7b013cb 100644
--- a/src/kernel_space.rs
+++ b/src/kernel_space.rs
@@ -139,6 +139,11 @@
         (size + Self::PAGE_SIZE - 1) & !(Self::PAGE_SIZE - 1)
     }
 
+    /// Returns the offset to the preceding page aligned address
+    pub const fn offset_in_page(address: usize) -> usize {
+        address & (Self::PAGE_SIZE - 1)
+    }
+
     /// Kernel virtual address to physical address
     #[cfg(not(test))]
     pub const fn kernel_to_pa(kernel_address: u64) -> u64 {