net/colo-compare.c: Fix ACK track reverse issue

The TCP protocol ACK maybe bigger than uint32_t MAX.
At this time, the ACK will reverse to 0. This patch
fix the max_ack and min_ack track issue.

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>