Revert "tcp: Set CHECKSUM_UNNECESSARY in tcp_init_nondata_skb"

This reverts commit 2626419a

.

It causes regressions for people with IGB cards.  Connection
requests don't complete etc.  The true cause of the issue is
still not known, but we should sort this out in net-next-2.6
not net-2.6

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