[Click] ipaddress and matches prefix

[Eddie Kohler]

Hi John,

> I was having trouble with some ndp stuff in ip6 with click,
> but I tracked the code down to ipaddress.hh:
> 
> It seems like matches_prefix should be something like matches_mask 
> instead..

I agree, it's a good fix (cleaner semantics), and I checked something in
that does the same thing.

Thanks!
XOXO
E