tmpfiles: downgrade errors when a file system does not support file attributes

tmpfiles: downgrade errors when a file system does not support file attributes

This downgrades errors from setting file attributes via tmpfiles to
warnings and makes them non-fatal.

Also, as a special case, if a file system does not support file
attributes at all, then the message is downgraded to debug, so that it
is not seen at all.

With this change reiserfs should not see any messages at all anymore
(since it apparently does not implement file attributes at all), but XFS
will still get a warning but no failure. The warning is something the
XFS kernel folks should fix though, by adjusting their file attributes
behaviour to be identical to ext234's.

Fixes #560.