xfs: use an XFS_OPSTATE_ flag for detecting if logged xattrs are available

Per reviewer request, use an OPSTATE flag (+ helpers) to decide if
logged xattrs are enabled, instead of querying the xfs_sb.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>