RFR: 8356606: PosixFileAttributes.permissions() implementations should return an EnumSet

[Brian Burkhalter]

Drop-in replacement in `sun.nio.fs.UnixFileAttributes::permissions` of `HashSet` with `EnumSet` for hopefully improved efficiency. All tests containing `PosixFileAttributes` passed.

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Commit messages:
 - 8356606: PosixFileAttributes.permissions() implementations should return an EnumSet

Changes: https://git.openjdk.org/jdk/pull/25162/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25162&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356606
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25162.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25162/head:pull/25162

PR: https://git.openjdk.org/jdk/pull/25162