xfs: report iomap_bn in block base

Report the iomap_bn field of struct xfs_iomap in terms of filesystem
blocks instead of in terms of bytes.  Shift the byte conversions
into the caller, and replace the IOMAP_DELAY and IOMAP_HOLE flag
checks with checks for HOLESTARTBLOCK and DELAYSTARTBLOCK.

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