RFR: 8318720: G1: Memory leak in G1CodeRootSet after JDK-8315503

[Zhengyu Gu]

Calling `ConcurrentHashTable:table.unsafe_reset()` in `G1CodeRootSet::clear()` does not free the entries that still in the table. This patch empties the table before the `unsafe_reset()` call to avoid memory leak.

Test:
 tier1 and tier2 (fastdebug and release) on Linux x86_64

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Commit messages:
 - Remove empty line
 - 8318720: G1: Memory leak in G1CodeRootSet after JDK-8315503

Changes: https://git.openjdk.org/jdk/pull/16352/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16352&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318720
  Stats: 19 lines in 1 file changed: 18 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16352.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16352/head:pull/16352

PR: https://git.openjdk.org/jdk/pull/16352