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>