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

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>