xfs: chain bios the right way around in xfs_rw_bdev

We need to chain the earlier bios to the later ones, so that
submit_bio_wait waits on the bio that all the completions are
dispatched to.

Fixes: 6ad5b325

 ("xfs: use bios directly to read and write the log recovery buffers")
Reported-by: Dave Chinner <david@fromorbit.com>
Tested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>