virtio_net: Clean up set_skb_frag()

Remove manual initialization in set_skb_frag, and instead
use __skb_fill_page_desc() to do the same. Patch tested
on net-next.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>