ipv6: Add GRO support

This patch adds GRO support for IPv6.  IPv6 GRO supports extension
headers in the same way as GSO (by using the same infrastructure).
It's also simpler compared to IPv4 since we no longer have to worry
about fragmentation attributes or header checksums.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>