hv_netvsc: Resize some of the variables in hv_netvsc_packet

As part of reducing the size of the hv_netvsc_packet, resize some of the
variables based on their usage.

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