xfs: factor out a xfs_defer_create_intent helper

Create a helper that encapsulates the whole logic to create a defer
intent.  This reorders some of the work that was done, but none of
that has an affect on the operation as only fields that don't directly
interact are affected.

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>