Decorate in6_clearscope() with __noubsan

sys/netinet6/scope6.c:480:6,
member access within misaligned address 0xffff9457bc441286 for type
'struct in6_addr' which requires 4 byte alignment

This issue is caused by accessing non-__packed struct in __packed.
This is a[always?] false-positive reported by the sanitizer and there is no
clear non-invasive approach to handle this, without changing ABI of long
term existing code.

Reported-by: syzbot+b53a9bcf030288081e65@syzkaller.appspotmail.com

sys/netinet6/scope6.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: scope6.c,v 1.20 2018/05/01 07:21:39 maxv Exp $	*/
+/*	$NetBSD: scope6.c,v 1.21 2019/09/20 14:53:24 kamil Exp $	*/
 /*	$KAME$	*/
 
 /*
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: scope6.c,v 1.20 2018/05/01 07:21:39 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: scope6.c,v 1.21 2019/09/20 14:53:24 kamil Exp $");
 
 #include <sys/param.h>
 #include <sys/malloc.h>
@@ -472,6 +472,7 @@ in6_getscopename(const struct in6_addr *addr)
  * Just clear the embedded scope identifier.  Return 0 if the original address
  * is intact; return non 0 if the address is modified.
  */
+__noubsan
 int
 in6_clearscope(struct in6_addr *in6)
 {