RFR: 8274562: (fs) UserDefinedFileAttributeView doesn't correctly determine if supported when using OverlayFS

[Brian Burkhalter]

Add extended attributes to capabilities flag if `_SYS_XATTR_H` is defined.

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Commit messages:
 - 8274562: (fs) UserDefinedFileAttributeView doesn't correctly determine if supported when using OverlayFS

Changes: https://git.openjdk.java.net/jdk/pull/5782/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5782&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274562
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5782.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5782/head:pull/5782

PR: https://git.openjdk.java.net/jdk/pull/5782