udf: reduce stack usage of udf_get_filename

Allocate strings with kmalloc.

Checkstack output:
Before: udf_get_filename:          600
After:  udf_get_filename:          136

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