tcp6: Add GRO support

This patch adds GRO support for TCP over IPv6.  The code is exactly
the same as the IPv4 version except for the pseudo-header checksum
computation.

Note that I've removed the unused tcphdr argument from tcp_v6_check
rather than invent a bogus value for GRO.

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