mm/shmem: shmem_replace_page() remember NR_SHMEM

Elsewhere, NR_SHMEM is updated at the same time as shmem NR_FILE_PAGES;
but shmem_replace_page() was forgetting to do that - so NR_SHMEM stats
could grow too big or too small, in those unusual cases when it's used.

Link: https://lkml.kernel.org/r/cec7c09d-5874-e160-ada6-6e10ee48784@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: Radoslaw Burny <rburny@google.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Hugh Dickins

mm/shmem.c

@@ -1659,7 +1659,9 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
 		new = page_folio(newpage);
 		mem_cgroup_migrate(old, new);
 		__inc_lruvec_page_state(newpage, NR_FILE_PAGES);
+		__inc_lruvec_page_state(newpage, NR_SHMEM);
 		__dec_lruvec_page_state(oldpage, NR_FILE_PAGES);
+		__dec_lruvec_page_state(oldpage, NR_SHMEM);
 	}
 	xa_unlock_irq(&swap_mapping->i_pages);