smp: Initialize arg->cpus sooner in smp_rendezvous_cpus_retry()

Otherwise, if !smp_started is true, then smp_rendezvous_cpus_done() will
harmlessly perform an atomic RMW on an uninitialized variable.

Reported by: KMSAN
Sponsored by: The FreeBSD Foundation

(cherry picked from commit cdfcfc607acc10c8b1b2b6a427c9e1ed7e94ebb3)