[jdk17u-dev] RFR: 8283469: Don't use memset to initialize members in FileMapInfo and fix memory leak

[Zhengyu Gu]

I would like to backport this patch to 17u, to fix a memory leak, also eliminates dangerous memset call, could potentially result in fatal crash.

The original patch does not apply cleanly, due to JDK-8261455, where it changed FileMapInfo's constructor signature to pass in full_path as a parameter. The conflict is resolved manually by initializing _full_path to nullptr.

I also bought down following code from JDK-8261455, given it is a P4 enhancement, I don't see it is a candidate for backport itself. But following code obviously fixes a bug that leaves file open after use. 

``` 
  if (_file_open) {
    os::close(_fd);
  }


Test:
  - [x] hotspot_cds on Linux x86_64
  - [ ]  GHA

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

Commit messages:
 - Backport d6fa8b004bcd0a2fc1015055d0177428889b4c31

Changes: https://git.openjdk.java.net/jdk17u-dev/pull/397/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u-dev&pr=397&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283469
  Stats: 12 lines in 1 file changed: 7 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk17u-dev/pull/397.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u-dev pull/397/head:pull/397

PR: https://git.openjdk.java.net/jdk17u-dev/pull/397