virtio: vhost gso is broken in some topology

virtio: vhost gso is broken in some topology

Recent modification added a call to vnet_gso_header_offset_parser in the
beginning of vhost_user_handle_tx_offload. The former routine may set tcp or
udp->checksum to 0. While it is appropriate to set it to 0 for the GSO packet,
it is broken and causes checksum error if the aformentiooned routine is called
by a non-GSO packet. The fix is to not call vhost_user_handle_tx_offload
if the buffer does not indicate checksum offload is needed.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I6e699d7a40b7887ff149cd8f77e8f0fa9374ef19