cpufreq: governor: Use common mutex for dbs_data protection

Every governor relying on the common code in cpufreq_governor.c
has to provide its own mutex in struct common_dbs_data.  However,
there actually is no need to have a separate mutex per governor
for this purpose, they may be using the same global mutex just
fine.  Accordingly, introduce a single common mutex for that and
drop the mutex field from struct common_dbs_data.

That at least will ensure that the mutex is always present and
initialized regardless of what the particular governors do.

Another benefit is that the common code does not need a pointer to
a governor-related structure to get to the mutex which sometimes
helps.

Finally, it makes the code generally easier to follow.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Saravana Kannan <skannan@codeaurora.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/cpufreq_governor.h

@@ -228,11 +228,6 @@ struct common_dbs_data {
 
 	/* Governor specific ops, see below */
 	void *gov_ops;
-
-	/*
-	 * Protects governor's data (struct dbs_data and struct common_dbs_data)
-	 */
-	struct mutex mutex;
 };
 
 /* Governor Per policy data */
@@ -277,6 +272,7 @@ static ssize_t show_sampling_rate_min_gov_pol				\
 	return sprintf(buf, "%u\n", dbs_data->min_sampling_rate);	\
 }
 
+extern struct mutex dbs_data_mutex;
 extern struct mutex cpufreq_governor_lock;
 void dbs_check_cpu(struct dbs_data *dbs_data, int cpu);
 int cpufreq_governor_dbs(struct cpufreq_policy *policy,