rust: alloc: separate aligned_size from krealloc_aligned

Separate `aligned_size` from `krealloc_aligned`.

Subsequent patches implement `Allocator` derivates, such as `Kmalloc`,
that require `aligned_size` and replace the original `krealloc_aligned`.

Reviewed-by: Alice Ryhl <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Danilo Krummrich <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Danilo Krummrich

rust/kernel/alloc/allocator.rs

@@ -8,20 +8,25 @@ use core::ptr;
 
 struct KernelAllocator;
 
+/// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
+fn aligned_size(new_layout: Layout) -> usize {
+    // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
+    let layout = new_layout.pad_to_align();
+
+    // Note that `layout.size()` (after padding) is guaranteed to be a multiple of `layout.align()`
+    // which together with the slab guarantees means the `krealloc` will return a properly aligned
+    // object (see comments in `kmalloc()` for more information).
+    layout.size()
+}
+
 /// Calls `krealloc` with a proper size to alloc a new object aligned to `new_layout`'s alignment.
 ///
 /// # Safety
 ///
 /// - `ptr` can be either null or a pointer which has been allocated by this allocator.
 /// - `new_layout` must have a non-zero size.
 pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: Flags) -> *mut u8 {
-    // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
-    let layout = new_layout.pad_to_align();
-
-    // Note that `layout.size()` (after padding) is guaranteed to be a multiple of `layout.align()`
-    // which together with the slab guarantees means the `krealloc` will return a properly aligned
-    // object (see comments in `kmalloc()` for more information).
-    let size = layout.size();
+    let size = aligned_size(new_layout);
 
     // SAFETY:
     // - `ptr` is either null or a pointer returned from a previous `k{re}alloc()` by the