xfs: use iomap fiemap implementation

Note that this removes support for the untested FIEMAP_FLAG_XATTR.  It
could be added relatively easily with iomap ops for the attr fork, but
without test coverage I don't feel safe doing this.

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