xfs: flush data dev on external log write

We incorrectly flush the log device instead of the data device when
trying to ensure metadata is correctly on disk before writing the
unmount record.

Fixes: eef983ff ("xfs: journal IO cache flush reductions")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>