md/md-bitmap: fix stats collection for external bitmaps

The bitmap_get_stats() function incorrectly returns -ENOENT for external
bitmaps.

Remove the external bitmap check as the statistics should be available
regardless of bitmap storage location.

Return -EINVAL only for invalid bitmap with no storage (neither in
superblock nor in external file).

Note: "bitmap_info.external" here refers to a bitmap stored in a separate
file (bitmap_file), not to external metadata.

Fixes: 8d28d0d ("md/md-bitmap: Synchronize bitmap_get_stats() with bitmap lifetime")
Signed-off-by: Zheng Qixing <zhengqixing@huawei.com>
Link: https://lore.kernel.org/linux-raid/20250403015322.2873369-1-zhengqixing@huaweicloud.com
Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Author: Zheng Qixing

drivers/md/md-bitmap.c

@@ -2357,9 +2357,8 @@ static int bitmap_get_stats(void *data, struct md_bitmap_stats *stats)
 
 	if (!bitmap)
 		return -ENOENT;
-	if (bitmap->mddev->bitmap_info.external)
-		return -ENOENT;
-	if (!bitmap->storage.sb_page) /* no superblock */
+	if (!bitmap->mddev->bitmap_info.external &&
+	    !bitmap->storage.sb_page)
 		return -EINVAL;
 	sb = kmap_local_page(bitmap->storage.sb_page);
 	stats->sync_size = le64_to_cpu(sb->sync_size);