virtio_net: fix use after free

commit 0b725a2c
    net: Remove ndo_xmit_flush netdev operation, use signalling instead.

added code that looks at skb->xmit_more after the skb has
been put in TX VQ. Since some paths process the ring and free the skb
immediately, this can cause use after free.

Fix by storing xmit_more in a local variable.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>