credit2: avoid picking a spurious idle unit when caps are used

credit2: avoid picking a spurious idle unit when caps are used

Commit 07b0eb5d0ef0 ("credit2: make sure we pick a runnable unit from the
runq if there is one") did not fix completely the problem of potentially
selecting a scheduling unit that will then not be able to run.

In fact, in case caps are used and the unit we are currently looking
at, during the runqueue scan, does not have enough budget for being run,
we should continue looking instead than giving up and picking the idle
unit.

Suggested-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>