RFR: 8257415: ZGC: Fix barrier_data types

[Stuart Monteith]

On Mon, 30 Nov 2020 12:42:00 GMT, Per Liden <pliden at openjdk.org> wrote:

> The `barrier_data` is an `uin8_t`, but we sometimes pass it around as an `int`. With this patch we always treat it as an `uint8_t`.

Looks good to me. (I'm not a reviewer).

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Marked as reviewed by smonteith (Author).

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