RFR: 8318720: G1: Memory leak in G1CodeRootSet after JDK-8315503 [v2]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Thomas' comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16352/files
  - new: https://git.openjdk.org/jdk/pull/16352/files/f98d3967..6469fc1f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16352&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16352&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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