iomap: renumber IOMAP_HOLE to 0

Instead of keeping a separate unnamed state for uninitialized iomaps,
renumber IOMAP_HOLE to zero so that an uninitialized iomap is treated
as a hole.

Suggested-by: Darrick J. Wong <darrick.wong@oracle.com>
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>