Eliminate memory leak in FreeBSD devstat collector

The memory allocated by calloc was never freed. Since the devinfo struct
never leaves the function, anyway, we might as well just allocate it on
the stack.

collector/devstat_freebsd.go

@@ -64,11 +64,8 @@ typedef struct {
 
 int _get_ndevs() {
 	struct statinfo current;
-	int num_devices;
-
-	current.dinfo = (struct devinfo *)calloc(1, sizeof(struct devinfo));
-	if (current.dinfo == NULL)
-		return -2;
+	struct devinfo info = {};
+	current.dinfo = &info;
 
 	devstat_checkversion(NULL);
 
@@ -80,12 +77,11 @@ int _get_ndevs() {
 
 Stats _get_stats(int i) {
 	struct statinfo current;
-	int num_devices;
+	struct devinfo info = {};
+	current.dinfo = &info;
 
-	current.dinfo = (struct devinfo *)calloc(1, sizeof(struct devinfo));
 	devstat_getdevs(NULL, &current);
 
-	num_devices = current.dinfo->numdevs;
 	Stats stats;
 	uint64_t bytes_read, bytes_write, bytes_free;
 	uint64_t transfers_other, transfers_read, transfers_write, transfers_free;
@@ -186,9 +182,6 @@ func (c *devstatCollector) Update(ch chan<- prometheus.Metric) (err error) {
 	if count == -1 {
 		return errors.New("devstat_getdevs() failed")
 	}
-	if count == -2 {
-		return errors.New("calloc() failed")
-	}
 
 	for i := C.int(0); i < count; i++ {
 		stats := C._get_stats(i)