xfs: refactor xfs_file_iomap_begin_delay

Rejuggle the return path to prepare for filling out a source iomap.

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>