ipv6: Add IFA_F_DADFAILED flag

Add IFA_F_DADFAILED flag to denote an IPv6 address that has
failed Duplicate Address Detection, that way tools like
/sbin/ip can be more informative.

3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 2001:db8::1/64 scope global tentative dadfailed
       valid_lft forever preferred_lft forever
Signed-off-by: Brian Haley <brian.haley@hp.com>

ip/ipaddress.c

@@ -506,6 +506,10 @@ int print_addrinfo(const struct sockaddr_nl *who, struct nlmsghdr *n,
 		fprintf(fp, "dynamic ");
 	} else
 		ifa->ifa_flags &= ~IFA_F_PERMANENT;
+	if (ifa->ifa_flags&IFA_F_DADFAILED) {
+		ifa->ifa_flags &= ~IFA_F_DADFAILED;
+		fprintf(fp, "dadfailed ");
+	}
 	if (ifa->ifa_flags)
 		fprintf(fp, "flags %02x ", ifa->ifa_flags);
 	if (rta_tb[IFA_LABEL])