virtio_net: fix csum generation for virtio-net devices

The commit e858fae2 ("virtio_net: use common code for virtio_net_hdr
and skb GSO conversion") replaced the tun code for header manipulation
with the generic helpers. While doing so, it implictly moved the
skb_partial_csum_set() invocation after eth_type_trans(), which
invalidate the current gso start/offset values.
Fix it by moving the helper invocation before the mac pulling.

Fixes: e858fae2

 ("virtio_net: use common code for virtio_net_hdr and
skb GSO conversion")

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