[HTB]: rbtree cleanup

Add code to initialize rb tree nodes, and check for double deletion.
This is not a real fix, but I can make it trap sometimes and may
be a bandaid for: http://bugzilla.kernel.org/show_bug.cgi?id=6681



Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>