hv_netvsc: Fix the packet free when it is in skb headroom

In the two places changed, we now use netvsc_xmit_completion() which properly
frees hv_netvsc_packet in or not in skb headroom.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>