RFR: 8264821: DirectIOTest fails on a system with large block size

[Brian Burkhalter]

Please review this request to clean up a test. The main change is to ensure that the capacity passed to `ByteBuffer.allocateDirect()` is large enough that when `alignedSlice()` is called on the new buffer, it creates an aligned buffer which does not have size zero due due to position rounding resulting in out-of-bound values. Other changes in the test are cleanup.

-------------

Commit messages:
 - 8264821: DirectIOTest fails on a system with large block size

Changes: https://git.openjdk.java.net/jdk/pull/3482/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3482&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264821
  Stats: 33 lines in 1 file changed: 7 ins; 8 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3482.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3482/head:pull/3482

PR: https://git.openjdk.java.net/jdk/pull/3482