IP: Cleanups

Use symbols instead of magic constants while checking PMTU discovery
setsockopt.

Remove redundant test in ip_rt_frag_needed() (done by caller).

Signed-off-by: John Dykstra <john.dykstra1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -662,7 +662,7 @@ done:
 	case IPV6_MTU_DISCOVER:
 		if (optlen < sizeof(int))
 			goto e_inval;
-		if (val<0 || val>3)
+		if (val < IP_PMTUDISC_DONT || val > IP_PMTUDISC_PROBE)
 			goto e_inval;
 		np->pmtudisc = val;
 		retv = 0;