Add String() method to AddressMask

PiperOrigin-RevId: 216770391
Change-Id: Idcdc28b2fe9e1b0b63b8119d445f05a8bcbce81e

pkg/tcpip/tcpip.go

@@ -139,6 +139,11 @@ type Address string
 // AddressMask is a bitmask for an address.
 type AddressMask string
 
+// String implements Stringer.
+func (a AddressMask) String() string {
+	return Address(a).String()
+}
+
 // Subnet is a subnet defined by its address and mask.
 type Subnet struct {
 	address Address