net/vhost: perform SW checksum in Rx path

net/vhost: perform SW checksum in Rx path

Virtio specification supports host checksum offloading
for L4, which is enabled with VIRTIO_NET_F_CSUM feature
negotiation. However, the Vhost PMD does not advertise
Rx checksum offload capabilities, so we can end-up with
the VIRTIO_NET_F_CSUM feature being negotiated, implying
the Vhost library returns packets with checksum being
offloaded while the application did not request for it.

Advertising these offload capabilities at the ethdev level
is not enough, because we could still end-up with the
application not enabling these offloads while the guest
still negotiate them.

This patch advertises the Rx checksum offload capabilities,
and introduces a compatibility layer to cover the case
VIRTIO_NET_F_CSUM has been negotiated but the application
does not configure the Rx checksum offloads. This function
performis the L4 Rx checksum in SW for UDP and TCP. Note
that it is not needed to calculate the pseudo-header
checksum, because the Virtio specification requires that
the driver do it.

This patch does not advertise SCTP checksum offloading
capability for now, but it could be handled later if the
need arises.

Reported-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Cheng Jiang <cheng1.jiang@intel.com>