rtnetlink: provide RCU protection to rtnl_fill_prop_list()

We want to be able to run rtnl_fill_ifinfo() under RCU protection
instead of RTNL in the future.

dev->name_node items are already rcu protected.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>