cpufreq: OMAP: use RCU locks around usage of OPP

OPP pointer is RCU protected, hence after finding it, de-reference
also should be protected with the same RCU context else the OPP
pointer may become invalid.
Reported-by: NAlexander Holler <holler@ahsoftware.de>
Tested-by: NAlexander Holler <holler@ahsoftware.de>
Acked-by: NAlexander Holler <holler@ahsoftware.de>
Signed-off-by: NNishanth Menon <nm@ti.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/omap-cpufreq.c

@@ -110,13 +110,16 @@ static int omap_target(struct cpufreq_policy *policy,
 	freq = ret;
 
 	if (mpu_reg) {
+		rcu_read_lock();
 		opp = opp_find_freq_ceil(mpu_dev, &freq);
 		if (IS_ERR(opp)) {
+			rcu_read_unlock();
 			dev_err(mpu_dev, "%s: unable to find MPU OPP for %d\n",
 				__func__, freqs.new);
 			return -EINVAL;
 		}
 		volt = opp_get_voltage(opp);
+		rcu_read_unlock();
 		tol = volt * OPP_TOLERANCE / 100;
 		volt_old = regulator_get_voltage(mpu_reg);
 	}