RFR: 8278172: java/nio/channels/FileChannel/BlockDeviceSize.java should only run on Linux

[Brian Burkhalter]

Please review this simple change which merely adds `@requires (os.family == "linux")` to the test.

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Commit messages:
 - 8278172: java/nio/channels/FileChannel/BlockDeviceSize.java should only run on Linux

Changes: https://git.openjdk.java.net/jdk/pull/6681/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6681&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278172
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6681.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6681/head:pull/6681

PR: https://git.openjdk.java.net/jdk/pull/6681