mm/page_owner: use order instead of nr in split_page_owner()

mm/page_owner: use order instead of nr in split_page_owner()

We do not have non power of two pages, using nr is error prone if nr is
not power-of-two.  Use page order instead.

Link: https://lkml.kernel.org/r/20230403201839.4097845-3-zi.yan@sent.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>