net: hold rtnl again in dump callbacks

Commit e67f88dd12f6 (dont hold rtnl mutex during netlink dump callbacks)
missed fact that rtnl_fill_ifinfo() must be called with rtnl held.

Because of possible deadlocks between two mutexes (cb_mutex and rtnl),
its not easy to solve this problem, so revert this part of the patch.

It also forgot one rcu_read_unlock() in FIB dump_rules()

Add one ASSERT_RTNL() in rtnl_fill_ifinfo() to remind us the rule.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Patrick McHardy <kaber@trash.net>
CC: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/fib_rules.c

diff --git a/net/core/fib_rules.c b/net/core/fib_rules.c
index 3911586..008dc70 100644
--- a/net/core/fib_rules.c
+++ b/net/core/fib_rules.c
@@ -602,6 +602,7 @@
 skip:
 		idx++;
 	}
+	rcu_read_unlock();
 	cb->args[1] = idx;
 	rules_ops_put(ops);