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

[Florian Weimer]

* DJ Delorie via Libc-alpha:

> From: Sergey <s.korolev@ndmsystems.com>
> Date: Fri, 24 Apr 2020 17:18:41 -0400
> Subject: Use unsigned constants for ICMP6 filters [BZ #22489]
>
> The core problem here is that the bitfields are unsigned but
> the computed constants are signed.  This patch forces the computed
> constants to be unsigned. - DJ

I suggest to write “the icmp6_filt array elements are unsigned”
instead of “the bitfields are unsigned”.