netvsc: fix rcu dereference warning from ethtool

The ethtool info command calls the netvsc get_sset_count with RTNL
but not with RCU. Which causes warning:

drivers/net/hyperv/netvsc_drv.c:1010 suspicious rcu_dereference_check() usage!

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