xen/netfront: fix inbound packet flags for checksum offload

Currently netfront is setting the flags of inbound packets with the checksum
not present (offloaded) to (CSUM_IP_CHECKED | CSUM_IP_VALID | CSUM_DATA_VALID |
CSUM_PSEUDO_HDR). According to the mbuf(9) man page this is not the correct
combination of flags, it should instead be (CSUM_DATA_VALID |
CSUM_PSEUDO_HDR).

Reviewed by: Wei Liu <wei.liu2@citrix.com>
MFC after: 2 weeks
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D9831