netvsc: use RCU to protect inner device structure

The netvsc driver has an internal structure (netvsc_device) which
is created when device is opened and released when device is closed.
And also opened/released when MTU or number of channels change.

Since this is referenced in the receive and transmit path, it is
safer to use RCU to protect/prevent use after free problems.

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