RFR: 8318720: G1: Memory leak in G1CodeRootSet after JDK-8315503 [v3]

[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:

  Adopt Thomas' code

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16352/files
  - new: https://git.openjdk.org/jdk/pull/16352/files/6469fc1f..205300b3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16352&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16352&range=01-02

  Stats: 19 lines in 1 file changed: 0 ins; 14 del; 5 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