ipv6: Add GSO support on forwarding path

Currently we disallow GSO packets on the IPv6 forward path.
This patch fixes this.

Note that I discovered that our existing GSO MTU checks (e.g.,
IPv4 forwarding) are buggy in that they skip the check altogether,
when they really should be checking gso_size + header instead.

I have also been lazy here in that I haven't bothered to segment
the GSO packet by hand before generating an ICMP message.  Someone
should add that to be 100% correct.

Reported-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>