datapath: STT: Fix checksum handling.

On packet receive STT verifies the checksum if not done in
hardware. But IP and TCP were pulled before the verification
step. The verification expect to see packet with TCP header.
This causes STT to drop packet in certain cases.

Signed-off-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>