RFR: 8315739: WSL1 build crashes after JDK-8312018

[Daniel Jeliński]

Please review this fix for handling of file `/proc/sys/vm/mmap_min_addr` which does not exist on WSL1.

This fixes a crash if the file is absent, and a file handle leak (missing `fclose`) when the file is present.

Linux tier1 tests clean.

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Commit messages:
 - Fix file handling

Changes: https://git.openjdk.org/jdk/pull/15596/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15596&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315739
  Stats: 9 lines in 1 file changed: 6 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15596.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15596/head:pull/15596

PR: https://git.openjdk.org/jdk/pull/15596