udf: Fix handling of multisession media

According to OSTA UDF specification, only anchor blocks and primary volume
descriptors are placed on media relative to the last session. All other block
numbers are absolute (in the partition or the whole media). This seems to be
confirmed by multisession media created by other systems.

Signed-off-by: Sebastian Manciulea <manciuleas@yahoo.com>
Signed-off-by: Jan Kara <jack@suse.cz>