netvsc: don't overload variable in same function

There are two variables named packet in the same function. One is the
metadata descriptor from host (vmpacket_descriptor) and the other is
the control block in the skb used to hold metadata from send.
Change name to avoid possible confusion and bugs.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>