cpufreq: interactive: fix policy locking

cpufreq_interactive_speedchange_task() is running as a separate kernel
thread and is calling __cpufreq_driver_target(), which requires callers
to hold policy->rwsem for writing to prevent racing with other parts of
the kernel trying to adjust the frequency, for example kernel thermal
throttling. Let's change the code to take policy->rwsem and while at it
refactor the code a bit.

This was originally 2 changes reviewed at:
	https://chromium-review.googlesource.com/246273
	https://chromium-review.googlesource.com/256120



Change-Id: Icc2d97c6c1b929acd2ee32e8c81d81fd2af778ab
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@google.com>