virtio_net: missing sg_init_table

Add missing sg_init_table for sg_set_buf in virtio_net which
induced in defer skb patch.

Reported-by: Thomas Müller <thomas@mathtm.de>
Tested-by: Thomas Müller <thomas@mathtm.de>
Signed-off-by: Shirley Ma <xma@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>