btrfs: subpage: fix the bitmap dump which can cause bitmap corruption

In commit 75258f20 ("btrfs: subpage: dump extra subpage bitmaps for
debug") an internal macro GET_SUBPAGE_BITMAP() is introduced to grab the
bitmap of each attribute.

But that commit is using bitmap_cut() which will do the left shift of
the larger bitmap, causing incorrect values.

Thankfully this bitmap_cut() is only called for debug usage, and so far
it's not yet causing problem.

Fix it to use bitmap_read() to only grab the desired sub-bitmap.

Fixes: 75258f20 ("btrfs: subpage: dump extra subpage bitmaps for debug")
CC: stable@vger.kernel.org # 6.6+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/subpage.c

@@ -902,8 +902,14 @@ void btrfs_folio_end_all_writers(const struct btrfs_fs_info *fs_info, struct fol
 }
 
 #define GET_SUBPAGE_BITMAP(subpage, subpage_info, name, dst)		\
-	bitmap_cut(dst, subpage->bitmaps, 0,				\
-		   subpage_info->name##_offset, subpage_info->bitmap_nr_bits)
+{									\
+	const int bitmap_nr_bits = subpage_info->bitmap_nr_bits;	\
+									\
+	ASSERT(bitmap_nr_bits < BITS_PER_LONG);				\
+	*dst = bitmap_read(subpage->bitmaps,				\
+			   subpage_info->name##_offset,			\
+			   bitmap_nr_bits);				\
+}
 
 void __cold btrfs_subpage_dump_bitmap(const struct btrfs_fs_info *fs_info,
 				      struct folio *folio, u64 start, u32 len)