kernel: initialize and free cpufreq stats properly

Initialize task's cpufreq to NULL including for idle
Make sure free task's cpufreq when free task struct

[backported to 3.10: Corinna Vinschen <xda@vinschen.de>]

Bug: 110044919
Change-Id: Ie4629d0ebe3ef4b72dffea3ee613b15f40a57142
Signed-off-by: Wei Wang <wvw@google.com>