cpufreq / stats: fix race between stats allocation and first usage
This patch forces complete struct cpufreq_stats allocation for
all cpus before registering CPUFREQ_TRANSITION_NOTIFIER notifier,
otherwise in some conditions cpufreq_stat_notifier_trans() can
be called in the middle of stats allocation, in this case
cpufreq_stats_table already exists, but stat_freq_table is NULL.

(Offline cherry-pick - CvD)
franciscofranco.1990@gmail.com
khlebnikov@openvz.org
rafael.j.wysocki@intel.com