xfs: report iomap_offset and iomap_bsize in block base

Report the iomap_offset and iomap_bsize fields of struct xfs_iomap
in terms of fsblocks instead of in terms of disk blocks.  Shift the
byte conversions into the callers temporarily, but they will
disappear or get cleaned up later.

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