xfs: remove iomap_target

Instead of using the iomap_target field in struct xfs_iomap
and the IOMAP_REALTIME flag just use the already existing
xfs_find_bdev_for_inode helper.  There's some fallout as we
need to pass the inode in a few more places, which we also
use to sanitize some calling conventions.

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