xfs: devirtualize ->node_hdr_from_disk

Replace the ->node_hdr_from_disk dir ops method with a directly called
xfs_da_node_hdr_from_disk helper that takes care of the v4 vs v5
difference.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>