align kernel space down to next page boundary

- otherwise PageRange::new is able to return an error

src/arch/x86_64/mm/virtualmem.rs

@@ -27,7 +27,9 @@ pub fn init() {
 	} else {
 		PageRange::new(
 			mm::kernel_end_address().as_usize(),
-			kernel_heap_end().as_usize() + 1,
+			kernel_heap_end()
+				.as_usize()
+				.align_down(BasePageSize::SIZE as usize),
 		)
 		.unwrap()
 	};