RFR: 8341246: Test com/sun/tools/attach/PermissionTest.java fails access denied

[SendaoYan]

Hi all,
Test `com/sun/tools/attach/PermissionTest.java` fails access denied after [JDK-8327114](https://bugs.openjdk.org/browse/JDK-8327114). This testcase need the `readlink` permission of file `/proc/self/ns/mnt` after [JDK-8327114](https://bugs.openjdk.org/browse/JDK-8327114).
So this PR add `readlink` permission to make this test work normally.
Before this PR, test run failed, after this PR, test run success. Test fix only, no risk.

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Commit messages:
 - 8341246: Test com/sun/tools/attach/PermissionTest.java fails access denied

Changes: https://git.openjdk.org/jdk/pull/21269/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21269&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341246
  Stats: 4 lines in 1 file changed: 1 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21269.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21269/head:pull/21269

PR: https://git.openjdk.org/jdk/pull/21269