fib: add rtnl locking in ip_fib_net_exit

Daniel J Blueman reported a lockdep splat in trie_firstleaf(), caused by
RTNL being not locked before a call to fib_table_flush()

Reported-by: Daniel J Blueman <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/fib_frontend.c

···
 	fib4_rules_exit(net);
 #endif
 
+	rtnl_lock();
 	for (i = 0; i < FIB_TABLE_HASHSZ; i++) {
 		struct fib_table *tb;
 		struct hlist_head *head;
···
 			fib_free_table(tb);
 		}
 	}
+	rtnl_unlock();
 	kfree(net->ipv4.fib_table_hash);
 }