fq_codel: should use qdisc backlog as threshold

fq_codel: should use qdisc backlog as threshold

codel_should_drop() logic allows a packet being not dropped if queue
size is under max packet size.

In fq_codel, we have two possible backlogs : The qdisc global one, and
the flow local one.

The meaningful one for codel_should_drop() should be the global backlog,
not the per flow one, so that thin flows can have a non zero drop/mark
probability.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Dave Taht <[email protected]>
Cc: Kathleen Nichols <[email protected]>
Cc: Van Jacobson <[email protected]>
Signed-off-by: David S. Miller <[email protected]>