e1000e: Perform software segmentation for loopback

e1000e: Perform software segmentation for loopback

e1000e didn't perform software segmentation for loopback if virtio-net
header is enabled, which is wrong.

To fix the problem, introduce net_tx_pkt_send_custom(), which allows the
caller to specify whether offloading should be assumed or not.

net_tx_pkt_send_custom() also allows the caller to provide a custom
sending function. Packets with virtio-net headers and ones without
virtio-net headers will be provided at the same time so the function
can choose the preferred version. In case of e1000e loopback, it prefers
to have virtio-net headers as they allows to skip the checksum
verification if VIRTIO_NET_HDR_F_DATA_VALID is set.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>