RFR: 8315786 : [AIX] Build Disk Local Detection Issue with GNU-utils df on AIX

[Deepa Kumari]

Previously [JDK-8304364](https://github.com/openjdk/jdk/pull/13066/files) , the AIX build process raised complaints about the disk location detection, incorrectly determining that the build wasn't on a local disk. However, a partial fix introduced a new problem, the build process consistently reports that it's on a local disk, even when it's not
The core problem here seems to be that Bash treats quoted commands as string literals, and it only evaluates them if you use 'eval' directly.
 The  change ensure that Bash correctly evaluates the **DF** variable as a command.

Reported Issue : [JDK-8315786](https://bugs.openjdk.org/browse/JDK-8315786)

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Commit messages:
 - 8315786 : [AIX] Build Disk Local Detection Issue with GNU-utils df on AIX

Changes: https://git.openjdk.org/jdk/pull/15592/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15592&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315786
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15592.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15592/head:pull/15592

PR: https://git.openjdk.org/jdk/pull/15592