udf: refactor udf_current_aext() to handle error

As Jan suggested in links below, refactor udf_current_aext() to
differentiate between error, hit EOF and success, it now takes pointer to
etype to store the extent type, return 1 when getting etype success,
return 0 when hitting EOF and return -errno when err.

Link: https://lore.kernel.org/all/20240912111235.6nr3wuqvktecy3vh@quack3/


Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241001115425.266556-2-zhaomzhao@126.com