btrfs: use specialized page copying helpers in btrfs_clone_extent_buffer

The copy_page is usually optimized and can be faster than memcpy.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4720,9 +4720,9 @@ struct extent_buffer *btrfs_clone_extent_buffer(struct extent_buffer *src)
 		WARN_ON(PageDirty(p));
 		SetPageUptodate(p);
 		new->pages[i] = p;
+		copy_page(page_address(p), page_address(src->pages[i]));
 	}
 
-	copy_extent_buffer(new, src, 0, 0, src->len);
 	set_bit(EXTENT_BUFFER_UPTODATE, &new->bflags);
 	set_bit(EXTENT_BUFFER_DUMMY, &new->bflags);