cyclictest: Always print an err message if write of 0 to cpu-dma_latency fails

In set_latency_target() there are some paths that don't print an error
message even when a write of 0 to /dev/cpu_dma_latency fails.

This patch does the following
- always print an error message if the write to /dev/cpu_dma_latency
fails
- Fix the error check with the write call. (a return of 0 or -1 indicate
 problems
- rename ret to err since this function is void and returns no value
- use err_msg_n instead of printf (which also prints to stderr)

Signed-off-by: John Kacur <jkacur@redhat.com>

src/cyclictest/cyclictest.c

@@ -233,20 +233,30 @@ static int32_t latency_target_value = 0;
 static void set_latency_target(void)
 {
 	struct stat s;
-	int ret;
+	int err;
 
-	if (stat("/dev/cpu_dma_latency", &s) == 0) {
-		latency_target_fd = open("/dev/cpu_dma_latency", O_RDWR);
-		if (latency_target_fd == -1)
+	errno = 0;
+	err = stat("/dev/cpu_dma_latency", &s);
+	if (err == -1) {
+		err_msg_n(errno, "WARN: stat /dev/cpu_dma_latency failed");
 		return;
-		ret = write(latency_target_fd, &latency_target_value, 4);
-		if (ret == 0) {
-			printf("# error setting cpu_dma_latency to %d!: %s\n", latency_target_value, strerror(errno));
+	}
+
+	errno = 0;
+	latency_target_fd = open("/dev/cpu_dma_latency", O_RDWR);
+	if (latency_target_fd == -1) {
+		err_msg_n(errno, "WARN: open /dev/cpu_dma_latency");
+		return;
+	}
+
+	errno = 0;
+	err = write(latency_target_fd, &latency_target_value, 4);
+	if (err < 1) {
+		err_msg_n(errno, "# error setting cpu_dma_latency to %d!", latency_target_value);
 		close(latency_target_fd);
 		return;
 	}
 	printf("# /dev/cpu_dma_latency set to %dus\n", latency_target_value);
-	}
 }