vtnet: move offload functions to virtio_net.h

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of an outdated version.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This patch removes the outdated copy from virtio_net.h and moves the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.

Reviewed by: tuexen
MFC after: 1 month
MFC to: stable/15
Differential Revision: https://reviews.freebsd.org/D57299