xfs: cleanup xfs_direct_write_iomap_begin

Move more checks into the helpers that determine if we need a COW
operation or allocation and split the return path for when an existing
data for allocation has been found versus a new allocation.

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>