net/virtio: fix Tx checksum offloads

net/virtio: fix Tx checksum offloads

Missing parenthesis around expression before type cast to struct
virtio_net_hdr pointer makes the arithmetic to be in
sizeof(struct virtio_net_hdr) units.

Use rte_pktmbuf_mtod_offset() to fix the problem.

Type of head_size is changed to signed since some compilers bark
on unary minus applied to unsigned.

Fixes: 1ae55ad38e5e ("net/virtio: fix mbuf data and packet length mismatch")
Cc: stable@dpdk.org
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>