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: ab99a875 ("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>