iomap: switch iomap_bmap to use iomap_iter

Rewrite the ->bmap implementation based on iomap_iter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[djwong: restructure the loop to make its behavior a little clearer]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>