mm: shmem: fix the update of 'shmem_falloc->nr_unswapped'

The 'shmem_falloc->nr_unswapped' is used to record how many writepage
refused to swap out because fallocate() is allocating, but after shmem
supports large folio swap out, the update of 'shmem_falloc->nr_unswapped'
does not use the correct number of pages in the large folio, which may
lead to fallocate() not exiting as soon as possible.

Anyway, this is found through code inspection, and I am not sure whether
it would actually cause serious issues.

Link: https://lkml.kernel.org/r/f66a0119d0564c2c37c84f045835b870d1b2196f.1734593154.git.baolin.wang@linux.alibaba.com
Fixes: 809bc86 ("mm: shmem: support large folio swap out")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Baolin Wang

mm/shmem.c

@@ -1535,7 +1535,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 			    !shmem_falloc->waitq &&
 			    index >= shmem_falloc->start &&
 			    index < shmem_falloc->next)
-				shmem_falloc->nr_unswapped++;
+				shmem_falloc->nr_unswapped += nr_pages;
 			else
 				shmem_falloc = NULL;
 			spin_unlock(&inode->i_lock);