credit2: replace cpumask_first() uses

credit2: replace cpumask_first() uses

... with cpumask_any() or cpumask_cycle().

In one case this also allows elimination of a cpumask_empty() call,
and while doing this I also spotted a redundant use of
cpumask_weight(). (When running on big systems, operations on CPU masks
aren't cheap enough to use them carelessly.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>