virtio-net: rearrange and complete SG initialization
virtio-net: rearrange and complete SG initialization

SG initialization was missing for those kernel versions where
NETMAP_LINUX_VIRTIO_MULTI_QUEUE is not defined and
NETMAP_LINUX_VIRTIO_SG is defined.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap