fq_codel: fix memory limitation drift

memory_usage must be decreased in dequeue_func(), not in
fq_codel_dequeue(), otherwise packets dropped by Codel algo
are missing this decrease.

Also we need to clear memory_usage in fq_codel_reset()

Fixes: 95b58430 ("fq_codel: add memory limitation per queue")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>