mm/rmap: not necessary to mask off FOLIO_PAGES_MAPPED

At this point, we are in an if branch conditional on (nr <
ENTIRELY_MAPPED), and FOLIO_PAGES_MAPPED is equal to (ENTIRELY_MAPPED -
1). This means the upper bits are already cleared.

It is not necessary to mask it off.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Wei Yang <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Liam R. Howlett <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Harry Yoo <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/rmap.c

···
 				nr_pages = folio_large_nr_pages(folio);
 				if (level == PGTABLE_LEVEL_PMD)
 					nr_pmdmapped = nr_pages;
-				nr = nr_pages - (nr & FOLIO_PAGES_MAPPED);
+				nr = nr_pages - nr;
 				/* Raced ahead of another remove and an add? */
 				if (unlikely(nr < 0))
 					nr = 0;