btrfs: fix a compilation error if DEBUG is defined in btree_dirty_folio

[BUG]
After commit 72a69cd ("btrfs: subpage: pack all subpage bitmaps
into a larger bitmap"), the DEBUG section of btree_dirty_folio() would
no longer compile.

[CAUSE]
If DEBUG is defined, we would do extra checks for btree_dirty_folio(),
mostly to make sure the range we marked dirty has an extent buffer and
that extent buffer is dirty.

For subpage, we need to iterate through all the extent buffers covered
by that page range, and make sure they all matches the criteria.

However commit 72a69cd ("btrfs: subpage: pack all subpage bitmaps
into a larger bitmap") changes how we store the bitmap, we pack all the
16 bits bitmaps into a larger bitmap, which would save some space.

This means we no longer have btrfs_subpage::dirty_bitmap, instead the
dirty bitmap is starting at btrfs_subpage_info::dirty_offset, and has a
length of btrfs_subpage_info::bitmap_nr_bits.

[FIX]
Although I'm not sure if it still makes sense to maintain such code, at
least let it compile.

This patch would let us test the bits one by one through the bitmaps.

CC: stable@vger.kernel.org # 6.1+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -520,6 +520,7 @@ static bool btree_dirty_folio(struct address_space *mapping,
 		struct folio *folio)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(mapping->host->i_sb);
+	struct btrfs_subpage_info *spi = fs_info->subpage_info;
 	struct btrfs_subpage *subpage;
 	struct extent_buffer *eb;
 	int cur_bit = 0;
@@ -533,18 +534,19 @@ static bool btree_dirty_folio(struct address_space *mapping,
 		btrfs_assert_tree_write_locked(eb);
 		return filemap_dirty_folio(mapping, folio);
 	}
+
+	ASSERT(spi);
 	subpage = folio_get_private(folio);
 
-	ASSERT(subpage->dirty_bitmap);
-	while (cur_bit < BTRFS_SUBPAGE_BITMAP_SIZE) {
+	for (cur_bit = spi->dirty_offset;
+	     cur_bit < spi->dirty_offset + spi->bitmap_nr_bits;
+	     cur_bit++) {
 		unsigned long flags;
 		u64 cur;
-		u16 tmp = (1 << cur_bit);
 
 		spin_lock_irqsave(&subpage->lock, flags);
-		if (!(tmp & subpage->dirty_bitmap)) {
+		if (!test_bit(cur_bit, subpage->bitmaps)) {
 			spin_unlock_irqrestore(&subpage->lock, flags);
-			cur_bit++;
 			continue;
 		}
 		spin_unlock_irqrestore(&subpage->lock, flags);
@@ -557,7 +559,7 @@ static bool btree_dirty_folio(struct address_space *mapping,
 		btrfs_assert_tree_write_locked(eb);
 		free_extent_buffer(eb);
 
-		cur_bit += (fs_info->nodesize >> fs_info->sectorsize_bits);
+		cur_bit += (fs_info->nodesize >> fs_info->sectorsize_bits) - 1;
 	}
 	return filemap_dirty_folio(mapping, folio);
 }