virtio: vhost gso checksum error when both indirect and mrg_rxbuf are off

Turn on gso, turn off both indirect and mrg_rxbuf caused traffic received
and sent with checksum error. The problem is we are not mapping the hdr
correctly in the shared memory address.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I7ef3bc2755544167b0e624365988111b17399e89