xfs: factor out two helpers from xfs_bmapi_write

We want to be able to reuse them for the upcoming dedidcated delalloc
convert routine.

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>