RFR: 8265231: (fc) ReadDirect and WriteDirect tests fail after fix for JDK-8264821

[Brian Burkhalter]

Please review this request to fix a test build failure caused by removing the `isDirectIOSupportedByFS()` method from the test `DirectIOTest` but not removing its use in other tests.

This also addresses the comment [1] in PR [2] of [3].

[1] https://github.com/openjdk/jdk/pull/3482#discussion_r613362581
[2] https://github.com/openjdk/jdk/pull/3482
[3] https://bugs.openjdk.java.net/browse/JDK-8264821

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Commit messages:
 - 8265231: (fc) ReadDirect and WriteDirect tests fail after fix for JDK-8264821

Changes: https://git.openjdk.java.net/jdk/pull/3498/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3498&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265231
  Stats: 26 lines in 5 files changed: 0 ins; 17 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3498.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3498/head:pull/3498

PR: https://git.openjdk.java.net/jdk/pull/3498