udf: Simplify handling of Volume Descriptor Pointers

According to ECMA-167 3/8.4.2 Volume Descriptor Pointer is terminating
current extent of Volume Descriptor Sequence. Also according to ECMA-167
3/8.4.3 Volume Descriptor Sequence Number is not significant for Volume
Descriptor Pointers. Simplify the handling of Volume Descriptor Pointers
to take this into account.

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