BUG/MEDIUM: freq-ctr: Don't report overshoot for long inactivity period

BUG/MEDIUM: freq-ctr: Don't report overshoot for long inactivity period

The function returning the excess of events over the current period for a
target frequency (the overshoot) has a flaw if the inactivity period is too
long. In this case, the result may overflow. Instead to be negative, a very
high positive value is returned.

This function is used by the bandwidth limitation filter. It means after a
long inactivity period, a huge burst may be detected while it should not.

In fact, the problem arise from the moment we're past the current period. In
this case, we should not report any overshoot and just get the number of
remaining events as usual.

This patch should be backported as far as 2.7.