udf: Fix free space reporting for metadata and virtual partitions

Free space on filesystems with metadata or virtual partition maps
currently gets misreported. This is because these partitions are just
remapped onto underlying real partitions from which keep track of free
blocks. Take this remapping into account when counting free blocks as
well.

Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Reported-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>