xfs: make xfs_bmbt_to_iomap available outside of xfs_pnfs.c

And ensure it works for RT subvolume files an set the block device,
both of which will be needed to be able to use the function in the
buffered write path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>