Follow the RFC6980 and silently ignore following IPv6 NDP messages

that had the IPv6 fragmentation header:
o Neighbor Solicitation
o Neighbor Advertisement
o Router Solicitation
o Router Advertisement
o Redirect

Introduce M_FRAGMENTED mbuf flag, and set it after IPv6 fragment reassembly
is completed. Then check the presence of this flag in correspondig ND6
handling routines.

PR:		224247
MFC after:	2 weeks

newlib/libc/sys/rtems/include/netinet6/in6.h

@@ -658,6 +658,7 @@ struct ip6_mtuinfo {
 #define	M_LOOP			M_PROTO6
 #define	M_AUTHIPDGM		M_PROTO7
 #define	M_RTALERT_MLD		M_PROTO8
+#define	M_FRAGMENTED		M_PROTO9	/* contained fragment header */
 
 #ifdef _KERNEL
 struct cmsghdr;