[jdk11u-dev] RFR: 8278172: java/nio/channels/FileChannel/BlockDeviceSize.java should only run on Linux

[Zhengyu Gu]

A clean backport for parity with Oracle 11.0.15.

This is a test only patch, no risk.

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Commit messages:
 - Backport 0d938cedcaf55886058b94dc280a8f7181c79fdf

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/742/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=742&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/jdk11u-dev/pull/742.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/742/head:pull/742

PR: https://git.openjdk.java.net/jdk11u-dev/pull/742