collector/zfs: Prevent procfs integer underflow

Prevent integer underflow when parsing the `procfs` file as it used a
`ParseUint` to parse signed values.

Fixes: prometheus#2766
Signed-off-by: Pranshu Srivastava <rexagod@gmail.com>

collector/zfs_linux.go

@@ -163,12 +163,21 @@ func (c *zfsCollector) parseProcfsFile(reader io.Reader, fmtExt string, handler
 
 		// kstat data type (column 2) should be KSTAT_DATA_UINT64, otherwise ignore
 		// TODO: when other KSTAT_DATA_* types arrive, much of this will need to be restructured
-		if parts[1] == kstatDataUint64 || parts[1] == kstatDataInt64 {
-			key := fmt.Sprintf("kstat.zfs.misc.%s.%s", fmtExt, parts[0])
+		key := fmt.Sprintf("kstat.zfs.misc.%s.%s", fmtExt, parts[0])
+		switch parts[1] {
+		case kstatDataUint64:
 			value, err := strconv.ParseUint(parts[2], 10, 64)
 			if err != nil {
 				return fmt.Errorf("could not parse expected integer value for %q", key)
 			}
+			// Handle unsigned integer value.
+			handler(zfsSysctl(key), value)
+		case kstatDataInt64:
+			value, err := strconv.ParseInt(parts[2], 10, 64)
+			if err != nil {
+				return fmt.Errorf("could not parse expected integer value for %q", key)
+			}
+			// Handle signed integer value.
 			handler(zfsSysctl(key), value)
 		}
 	}