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_ondemand.c

@@ -249,7 +249,7 @@ static void update_sampling_rate(struct dbs_data *dbs_data,
 	/*
 	 * Lock governor so that governor start/stop can't execute in parallel.
 	 */
-	mutex_lock(&od_dbs_cdata.mutex);
+	mutex_lock(&dbs_data_mutex);
 
 	cpumask_copy(&cpumask, cpu_online_mask);
 
@@ -306,7 +306,7 @@ static void update_sampling_rate(struct dbs_data *dbs_data,
 		}
 	}
 
-	mutex_unlock(&od_dbs_cdata.mutex);
+	mutex_unlock(&dbs_data_mutex);
 }
 
 static ssize_t store_sampling_rate(struct dbs_data *dbs_data, const char *buf,
@@ -552,7 +552,6 @@ static struct common_dbs_data od_dbs_cdata = {
 	.gov_ops = &od_ops,
 	.init = od_init,
 	.exit = od_exit,
-	.mutex = __MUTEX_INITIALIZER(od_dbs_cdata.mutex),
 };
 
 static int od_cpufreq_governor_dbs(struct cpufreq_policy *policy,