hv_netvsc: fix rtnl locking in callback

The function get_netvsc_net_device had conditional locking. This was
unnecessary, incorrect, but harmless. It was unnecessary since the
code is only called from netlink netdev event callback where RTNL
is always acquired before the callbacks are run. It was incorrect
because of use of trylock and then continuing.
Fix by replacing with proper assertion.

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