udf: Fix free space reporting for metadata and virtual partitions

udf: Fix free space reporting for metadata and virtual partitions

[ Upstream commit a4a8b99ec819ca60b49dc582a4287ef03411f117 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>