RFR: 8265100: (fs) WindowsFileStore.hashCode() should read cached hash code once

[Brian Burkhalter]

Please review this small request to clean up `WindowsFileStore.hasCode()`.

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Commit messages:
 - 8265100: (fs) WindowsFileStore.hashCode() should read cached hash code once

Changes: https://git.openjdk.java.net/jdk/pull/3445/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3445&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265100
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3445.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3445/head:pull/3445

PR: https://git.openjdk.java.net/jdk/pull/3445