xfs: handle zeroing in xfs_file_iomap_begin_delay

We only need to allocate blocks for zeroing for reflink inodes,
and for we currently have a special case for reflink files in
the otherwise direct I/O path that I'd like to get rid of.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>