udf: convert udf_stamp_to_time and udf_time_to_stamp to use timestamps

* kernel_timestamp type was almost unused - only callers of udf_stamp_to_time
and udf_time_to_stamp used it, so let these functions handle endianness
internally and don't clutter code with conversions

* rename udf_stamp_to_time to udf_disk_stamp_to_time
  and udf_time_to_stamp to udf_time_to_disk_stamp

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>