xen: credit2: only reset credit on reset condition

xen: credit2: only reset credit on reset condition

The condition for a Credit2 scheduling epoch coming to an
end is that the vcpu at the front of the runqueue has negative
credits. However, it is possible, that runq_candidate() does
not actually return to the scheduler the first vcpu in the
runqueue (e.g., because such vcpu can't run on the cpu that
is going through the scheduler, because of hard-affinity).

If that happens, we should not trigger a credit reset, or we
risk altering the lenght of a scheduler epoch, wrt what the
original idea of the algorithm was.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>