[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>