tcp: Fix ipv6 checksumming on response packets for real.

Commit 6651ffc8
("ipv6: Fix tcp_v6_send_response transport header setting.")
fixed one half of why ipv6 tcp response checksums were
invalid, but it's not the whole story.

If we're going to use CHECKSUM_PARTIAL for these things (which we are
since commit 2e8e18ef "tcp: Set
CHECKSUM_UNNECESSARY in tcp_init_nondata_skb"), we can't be setting
buff->csum as we always have been here in tcp_v6_send_response.  We
need to leave it at zero.

Kill that line and checksums are good again.

Signed-off-by: David S. Miller <davem@davemloft.net>