ipvs, bug: quorum state was flapping when Q < H

When a service had quorum < hysteresis, the lower threshold of RS weights was calculated incorrecly.

Unsigned arythmetics was used, so errors like this appeared in log:
  Keepalived_healthcheckers[2535]: Lost quorum 1-2=18446744073709551615 > 10 for VS

The up -> down quorum state transition was happening every time when alive RS set was changed.

This bug was in place since keepalived-1.2.9