s390/cpum_sf: ensure sample frequency of perf event attributes is non-zero

Correct a trinity finding for the perf_event_open() system call with
a perf event attribute structure that uses a frequency but has the
sampling frequency set to zero.  This causes a FP divide exception during
the sample rate initialization for the hardware sampling facility.

Fixes: 8c069ff4 ("s390/perf: add support for the CPU-Measurement Sampling Facility")
Cc: stable@vger.kernel.org # 3.14+
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/perf_cpum_sf.c

@@ -753,6 +753,10 @@ static int __hw_perf_event_init(struct perf_event *event)
 	 */
 	rate = 0;
 	if (attr->freq) {
+		if (!attr->sample_freq) {
+			err = -EINVAL;
+			goto out;
+		}
 		rate = freq_to_sample_rate(&si, attr->sample_freq);
 		rate = hw_limit_rate(&si, rate);
 		attr->freq = 0;