udf: udf_CS0toUTF8 cleanup

- fix error handling - always zero output variable
- don't zero explicitely fields zeroed by memset
- mark "in" paramater as const
- remove outdated comment

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
diff --git a/fs/udf/udfdecl.h b/fs/udf/udfdecl.h
index d3aae56..2310b5c8 100644
--- a/fs/udf/udfdecl.h
+++ b/fs/udf/udfdecl.h
@@ -175,7 +175,7 @@
 extern int udf_put_filename(struct super_block *, const uint8_t *, uint8_t *,
 			    int);
 extern int udf_build_ustr(struct ustr *, dstring *, int);
-extern int udf_CS0toUTF8(struct ustr *, struct ustr *);
+extern int udf_CS0toUTF8(struct ustr *, const struct ustr *);
 
 /* ialloc.c */
 extern void udf_free_inode(struct inode *);