xfs: fold dfops into the transaction

struct xfs_defer_ops has now been reduced to a single list_head. The
external dfops mechanism is unused and thus everywhere a (permanent)
transaction is accessible the associated dfops structure is as well.

Remove the xfs_defer_ops structure and fold the list_head into the
transaction. Also remove the last remnant of external dfops in
xfs_trans_dup().

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