IPv6 support for hexCIDR()

Includes these changes:
- Modified so that IPv6 CIDRs can be converted correctly.
- Added test cases for IPv6 addresses.
- Split UTs for hexCIDR() and asciiCIDR() so that masking can be tested.
- Add UTs for failure cases.

Note: Some code that calls hexCIDR() builds a CIDR from the pod IP string
and the concatenation of "/32". These should, in the future, use "128",
if/when the pod IP is IPv6. Not addressed as part of this commit.

pkg/util/bandwidth/linux.go

@@ -101,7 +101,7 @@ func hexCIDR(cidr string) (string, error) {
 		return "", err
 	}
 	ip = ip.Mask(ipnet.Mask)
-	hexIP := hex.EncodeToString([]byte(ip.To4()))
+	hexIP := hex.EncodeToString([]byte(ip))
 	hexMask := ipnet.Mask.String()
 	return hexIP + "/" + hexMask, nil
 }
@@ -119,6 +119,9 @@ func asciiCIDR(cidr string) (string, error) {
 	ip := net.IP(ipData)
 
 	maskData, err := hex.DecodeString(parts[1])
+	if err != nil {
+		return "", err
+	}
 	mask := net.IPMask(maskData)
 	size, _ := mask.Size()