fib_trie: Fix trie balancing issue if new node pushes down existing node

This patch addresses an issue with the level compression of the fib_trie.
Specifically in the case of adding a new leaf that triggers a new node to
be added that takes the place of the old node.  The result is a trie where
the 1 child tnode is on one side and one leaf is on the other which gives
you a very deep trie.  Below is the script I used to generate a trie on
dummy0 with a 10.X.X.X family of addresses.

  ip link add type dummy
  ipval=184549374
  bit=2
  for i in `seq 1 23`
  do
    ifconfig dummy0:$bit $ipval/8
    ipval=`expr $ipval - $bit`
    bit=`expr $bit \* 2`
  done
  cat /proc/net/fib_triestat

Running the script before the patch:

	Local:
		Aver depth:     10.82
		Max depth:      23
		Leaves:         29
		Prefixes:       30
		Internal nodes: 27
		  1: 26  2: 1
		Pointers: 56
	Null ptrs: 1
	Total size: 5  kB

After applying the patch and repeating:

	Local:
		Aver depth:     4.72
		Max depth:      9
		Leaves:         29
		Prefixes:       30
		Internal nodes: 12
		  1: 3  2: 2  3: 7
		Pointers: 70
	Null ptrs: 30
	Total size: 4  kB

What this fix does is start the rebalance at the newly created tnode
instead of at the parent tnode.  This way if there is a gap between the
parent and the new node it doesn't prevent the new tnode from being
coalesced with any pre-existing nodes that may have been pushed into one
of the new nodes child branches.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1143,8 +1143,9 @@ static struct list_head *fib_insert_node(struct trie *t, u32 key, int plen)
 			put_child(tp, cindex, (struct rt_trie_node *)tn);
 		} else {
 			rcu_assign_pointer(t->trie, (struct rt_trie_node *)tn);
-			tp = tn;
 		}
+
+		tp = tn;
 	}
 
 	if (tp && tp->pos + tp->bits > 32)