ipaddress: do not add IFA_FLAGS when not necessary

commit 37c9b94e (add support for extended ifa_flags)
introduced a regression:

        # ./ip/ip addr add 192.168.0.1/24 dev eth0
        RTNETLINK answers: Invalid argument

This is due to old kernels don't support IFA_FLAGS flag, we should not
use it if we don't use the flags beyond old .ifa_flags.

Cc: Jiri Pirko <jiri@resnulli.us>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>

ip/ipaddress.c

@@ -1398,8 +1398,10 @@ static int ipaddr_modify(int cmd, int flags, int argc, char **argv)
 		}
 		argc--; argv++;
 	}
-	req.ifa.ifa_flags = ifa_flags;
-	addattr32(&req.n, sizeof(req), IFA_FLAGS, ifa_flags);
+	if (ifa_flags <= 0xff)
+		req.ifa.ifa_flags = ifa_flags;
+	else
+		addattr32(&req.n, sizeof(req), IFA_FLAGS, ifa_flags);
 
 	if (d == NULL) {
 		fprintf(stderr, "Not enough information: \"dev\" argument is required.\n");