[IPV6] IP6TUNNEL: Add support to IPv4 over IPv6 tunnel.

Some notes
- Protocol number IPPROTO_IPIP is used for IPv4 over IPv6 packets.
- If IP6_TNL_F_USE_ORIG_TCLASS is set, TOS in IPv4 header is copied to
  Traffic Class in outer IPv6 header on xmit.
- IP6_TNL_F_USE_ORIG_FLOWLABEL is ignored on xmit of IPv4 packets, because
  IPv4 header does not have flow label.
- Kernel sends ICMP error if IPv4 packet is too big on xmit, even if
  DF flag is not set.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>