xfs: turn dfp_intent into a xfs_log_item

xfs: turn dfp_intent into a xfs_log_item

Source kernel commit: 13a8333339072b8654c1d2c75550ee9f41ee15de

All defer op instance place their own extension of the log item into
the dfp_intent field.  Replace that with a xfs_log_item to improve type
safety and make the code easier to follow.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>