xfs: move xfs_file_iomap_begin_delay around

Move xfs_file_iomap_begin_delay near the end of the file next to the
other iomap functions to prepare for additional refactoring.
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>