RFR: 8257415: ZGC: Fix barrier_data types

[Per Liden]

On Tue, 1 Dec 2020 08:37:38 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> The `barrier_data` is an `uint8_t`, but we sometimes pass it around as an `int`. With this patch we always treat it as an `uint8_t`.
>
> Marked as reviewed by stefank (Reviewer).

Thanks for reviewing!

-------------

PR: https://git.openjdk.java.net/jdk/pull/1514