mm: free up a word in the first tail page

mm: free up a word in the first tail page

Store the folio order in the low byte of the flags word in the first tail
page.  This frees up the word that was being used to store the order and
dtor bytes previously.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: Yanteng Si <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>