syscallcompat: downgrade DT_UNKNOWN message level on XFS
syscallcompat: downgrade DT_UNKNOWN message level on XFS

Old XFS filesystems always return DT_UNKNOWN. Downgrade the message
to "info" level if we are on XFS.

Using the "warning" level means that users on old XFS filesystems
cannot run the test suite as it intentionally aborts on any
warnings.

Fixes https://github.com/rfjakob/gocryptfs/issues/267

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs