Move the functions vtnet_rxq_csum() and vtnet_txq_offload() and the subfunctions they call from if_vtnet.c to virtio_net.h. This allows us to call these functions from if_tuntap.c and if_ptnet.c.
virtio_net.h already contained a copy of these functions, but a copy of an outdated version. The functions evolved in if_vtnet.c. In if_vtnet.c, the copy has never been used because it increments counters in their own functions.
This patch removes the outdated copy from virtio_net.h and moves the new version of the functions from if_vtnet.c to virtio_net.h. if_tuntap.c, if_ptnet.c, and if_vtnet.c just call these functions, and if_vtnet.c increments its counters depending on the return value.