scsi: ufs: core: Only collect timestamps if monitoring is enabled

scsi: ufs: core: Only collect timestamps if monitoring is enabled

Every ktime_get() call in the hot path has a measurable impact on IOPS.
Hence, only collect timestamps if the monitoring functionality is
enabled.

See also commit 1d8613a23f3c ("scsi: ufs: core: Introduce HBA
performance monitor sysfs nodes").

Cc: Can Guo <cang@codeaurora.org>
Cc: Bean Huo <beanhuo@micron.com>
Cc: Daejun Park <daejun7.park@samsung.com>
Reviewed-by: Peter Wang <peter.wang@mediatek.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20250819153958.2255907-1-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>