md/md-bitmap: fix wrong bitmap_limit for clustermd when write sb

In clustermd, separate write-intent-bitmaps are used for each cluster
node:

0                    4k                     8k                    12k
-------------------------------------------------------------------
| idle                | md super            | bm super [0] + bits |
| bm bits[0, contd]   | bm super[1] + bits  | bm bits[1, contd]   |
| bm super[2] + bits  | bm bits [2, contd]  | bm super[3] + bits  |
| bm bits [3, contd]  |                     |                     |

So in node 1, pg_index in __write_sb_page() could equal to
bitmap->storage.file_pages. Then bitmap_limit will be calculated to
0. md_super_write() will be called with 0 size.
That means the first 4k sb area of node 1 will never be updated
through filemap_write_page().
This bug causes hang of mdadm/clustermd_tests/01r1_Grow_resize.

Here use (pg_index % bitmap->storage.file_pages) to make calculation
of bitmap_limit correct.

Fixes: ab99a87 ("md/md-bitmap: fix writing non bitmap pages")
Signed-off-by: Su Yue <glass.su@suse.com>
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Link: https://lore.kernel.org/linux-raid/20250303033918.32136-1-glass.su@suse.com
Signed-off-by: Yu Kuai <yukuai3@huawei.com>

drivers/md/md-bitmap.c

@@ -428,8 +428,8 @@ static int __write_sb_page(struct md_rdev *rdev, struct bitmap *bitmap,
 	struct block_device *bdev;
 	struct mddev *mddev = bitmap->mddev;
 	struct bitmap_storage *store = &bitmap->storage;
-	unsigned int bitmap_limit = (bitmap->storage.file_pages - pg_index) <<
-		PAGE_SHIFT;
+	unsigned long num_pages = bitmap->storage.file_pages;
+	unsigned int bitmap_limit = (num_pages - pg_index % num_pages) << PAGE_SHIFT;
 	loff_t sboff, offset = mddev->bitmap_info.offset;
 	sector_t ps = pg_index * PAGE_SIZE / SECTOR_SIZE;
 	unsigned int size = PAGE_SIZE;
@@ -438,7 +438,7 @@ static int __write_sb_page(struct md_rdev *rdev, struct bitmap *bitmap,
 
 	bdev = (rdev->meta_bdev) ? rdev->meta_bdev : rdev->bdev;
 	/* we compare length (page numbers), not page offset. */
-	if ((pg_index - store->sb_index) == store->file_pages - 1) {
+	if ((pg_index - store->sb_index) == num_pages - 1) {
 		unsigned int last_page_size = store->bytes & (PAGE_SIZE - 1);
 
 		if (last_page_size == 0)