RFR: 8315786: [AIX] Build Disk Local Detection Issue with GNU-utils df on AIX [v2]

[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)

Deepa Kumari has updated the pull request incrementally with one additional commit since the last revision:

  remove quotes

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15592/files
  - new: https://git.openjdk.org/jdk/pull/15592/files/510a327d..326ef647

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15592&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15592&range=00-01

  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