RFR: JDK-8312394: [linux] SIGSEGV if kernel was built without hugepage support

[Thomas Stuefe]

Somewhat trivial fixes. Hugepage detection crashes on systems whose kernels have no support compiled in for them.

- check the result of opendir() before using
- don't scan for hugepages at all if we don't have a default hugepage.

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Commit messages:
 - start

Changes: https://git.openjdk.org/jdk/pull/14936/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14936&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312394
  Stats: 22 lines in 1 file changed: 6 ins; 2 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/14936.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14936/head:pull/14936

PR: https://git.openjdk.org/jdk/pull/14936