[IPV6]: Accept -1 for IPV6_TCLASS

This patch should add support for -1 as "default" IPv6 traffic class,
as specified in IETF RFC3542 ยง6.5. 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>
diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 4332726..a5eaaf6 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -362,7 +362,7 @@
 		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 @@
 
 	case IPV6_TCLASS:
 		val = np->tclass;
+		if (val < 0)
+			val = 0;
 		break;
 
 	case IPV6_RECVTCLASS: