If copy_file_range performs a copy using splice, it converts holes to zeros. This effort primarily changes this behavior to create holes when it is possible. Even if copy_file_range() or clone_file_range() does not work for different mounted filesystems, We should be able to splice files if they do not belong the same super_block. Changes since v0: - [Amir] Carved out do_copy_file_range() which can be used by overlayfs -- Goldwyn
