Growfs updates the secondary superblocks using synchronous unlogged
buffer writes after committing the updates to the primary superblock.

Mark the transaction to the primary superblock as synchronous so that
we guarantee it is committed to disk before we update the secondary
superblocks.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>