tcp_cubic: fix comparison of jiffies

Jiffies wraps around therefore the correct way to compare is
to use cast to signed value.

Note: cubic is not using full jiffies value on 64 bit arch
because using full unsigned long makes struct bictcp grow too
large for the available ca_priv area.

Includes correction from Sangtae Ha to improve ack train detection.

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