scsi: ufs: core: Fix data race in CPU latency PM QoS request handling

scsi: ufs: core: Fix data race in CPU latency PM QoS request handling

The cpu_latency_qos_add/remove/update_request interfaces lack internal
synchronization by design, requiring the caller to ensure thread safety.
The current implementation relies on the 'pm_qos_enabled' flag, which is
insufficient to prevent concurrent access and cannot serve as a proper
synchronization mechanism. This has led to data races and list
corruption issues.

A typical race condition call trace is:

[Thread A]
ufshcd_pm_qos_exit()
  --> cpu_latency_qos_remove_request()
    --> cpu_latency_qos_apply();
      --> pm_qos_update_target()
        --> plist_del              <--(1) delete plist node
    --> memset(req, 0, sizeof(*req));
  --> hba->pm_qos_enabled = false;

[Thread B]
ufshcd_devfreq_target
  --> ufshcd_devfreq_scale
    --> ufshcd_scale_clks
      --> ufshcd_pm_qos_update     <--(2) pm_qos_enabled is true
        --> cpu_latency_qos_update_request
          --> pm_qos_update_target
            --> plist_del          <--(3) plist node use-after-free

Introduces a dedicated mutex to serialize PM QoS operations, preventing
data races and ensuring safe access to PM QoS resources, including sysfs
interface reads.

Fixes: 2777e73fc154 ("scsi: ufs: core: Add CPU latency QoS support for UFS driver")
Signed-off-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Huan Tang <tanghuan@vivo.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>