udf: replace udf_*_offset macros with functions

- translate udf_file_entry_alloc_offset macro into function
- translate udf_ext0_offset macro into function
- add comment about crypticly named fields in struct udf_inode_info

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