mm: shmem: fix minor off-by-one in shrinkable calculation

There has been a long-standing and very minor off-by-one, where
shmem_get_folio_gfp() decides if a large folio extends beyond i_size far
enough to leave a page or more for freeing later under pressure.

This is not something needed for stable: but it will be proportionately
more significant as support for smaller large folios is added, and is best
fixed before duplicating the check in other places.

Link: https://lkml.kernel.org/r/d8e75079-af2d-8519-56df-6be1dccc247a@google.com
Fixes: 779750d2 ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shmem.c

@@ -2348,7 +2348,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	alloced = true;
 	if (folio_test_large(folio) &&
 	    DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
-					folio_next_index(folio) - 1) {
+					folio_next_index(folio)) {
 		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 		struct shmem_inode_info *info = SHMEM_I(inode);
 		/*