hv_netvsc: fix locking for rx_mode

The rx_mode operation handler is different than other callbacks
in that is not always called with rtnl held. Therefore use
RCU to ensure that references are valid.

Fixes: bee9d41b ("hv_netvsc: propagate rx filters to VF")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>