IpSetEntry how to set 0.0.0.0/0

Because of this code in `buildEntryData` func in `ipset_linux.go`
```
if entry.CIDR != 0 {
		data.AddChild(nl.NewRtAttr(nl.IPSET_ATTR_CIDR, nl.Uint8Attr(entry.CIDR)))
	}
```
If set 0 to entry.CIDR, it will be ignore.