[bufferbloat] Codel: avoid a nul rec_inv_sqrt

[bufferbloat] Codel: avoid a nul rec_inv_sqrt

One condition before codel_Newton_step() was not good if
we never left the dropping state for a flow. As a result
rec_inv_sqrt was 0, instead of the ~0 initial value.

codel control law was then set to a very aggressive mode, dropping
many packets before reaching 'target' and recovering from this problem.

Brought over from 3.5-stable

Signed-off-by: Dave Taht <dave.taht@bufferbloat.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@32950 3c298f89-4303-0410-b956-a3cf2f4a3e73