credit2: make sure we pick a runnable unit from the runq if there is one

credit2: make sure we pick a runnable unit from the runq if there is one

A !runnable unit (temporarily) present in the runq may cause us to
stop scanning the runq itself too early. Of course, we don't run any
non-runnable vCPUs, but we end the scan and we fallback to picking
the idle unit. In other word, this prevent us to find there and pick
the actual unit that we're meant to start running (which might be
further ahead in the runq).

Depending on the vCPU pinning configuration, this may lead to such
unit to be stuck in the runq for long time, causing malfunctioning
inside the guest.

Fix this by checking runnable/non-runnable status up-front, in the runq
scanning function.

Reported-by: Michał Leszczyński <michal.leszczynski@cert.pl>
Reported-by: Dion Kant <g.w.kant@hunenet.nl>
Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>