[patch] Use unsigned constants for ICMP6 filters [BZ #22489]

[Florian Weimer]

* DJ Delorie:

> The core problem here is that the filter array is unsigned but the
> computed constants are signed.  This both causes a signededness
> conversion at the &= step and may cause undefined behavior if the MSB
> is being modified.  This patch uses unsigned constants to avoid both
> cases. - DJ

I'd say that the filter array *elements* are unsigned, but I don't
feel strongly about that.