[IPV6]: Accept -1 for IPV6_TCLASS

This patch should add support for -1 as "default" IPv6 traffic class,
as specified in IETF RFC3542 §6.5. Within the kernel, it seems tclass
< 0 is already handled, but setsockopt, getsockopt and recvmsg calls
won't accept it from userland.
Signed-off-by: Remi Denis-Courmont <rdenis@simphalempin.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/datagram.c

@@ -696,7 +696,7 @@ int datagram_send_ctl(struct msghdr *msg, struct flowi *fl,
 			}
 
 			tc = *(int *)CMSG_DATA(cmsg);
-			if (tc < 0 || tc > 0xff)
+			if (tc < -1 || tc > 0xff)
 				goto exit_f;
 
 			err = 0;

net/ipv6/ipv6_sockglue.c

@@ -362,7 +362,7 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		break;
 
 	case IPV6_TCLASS:
-		if (val < 0 || val > 0xff)
+		if (val < -1 || val > 0xff)
 			goto e_inval;
 		np->tclass = val;
 		retv = 0;
@@ -947,6 +947,8 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 
 	case IPV6_TCLASS:
 		val = np->tclass;
+		if (val < 0)
+			val = 0;
 		break;
 
 	case IPV6_RECVTCLASS: