virtio_net: introduce virtio_net_hdr_{from,to}_skb

The code for conversion between virtio_net_hdr and skb GSO info is
duplicated at several places. Let's put it to a common place to allow
reuse.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>