xfs: kill struct xfs_iomap

Now that struct xfs_iomap contains exactly the same units as struct
xfs_bmbt_irec we can just use the latter directly in the aops code.
Replace the missing IOMAP_NEW flag with a new boolean output
parameter to xfs_iomap.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>