RFR: 8257415: ZGC: Fix barrier_data types

[Per Liden]

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`.

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Commit messages:
 - 8257415: ZGC: Fix barrier_data types

Changes: https://git.openjdk.java.net/jdk/pull/1514/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1514&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257415
  Stats: 7 lines in 4 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1514.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1514/head:pull/1514

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