src/t_open_tmpfiles: flush log when shutting down filesystem

src/t_open_tmpfiles: flush log when shutting down filesystem

If the caller of t_open_tmpfiles wants to shut down the filesystem,
be sure to flush the log when we shut down so that log recovery will
have to process all the unlinked temporary files.

This is apparently needed to force ext4 to flush updated inode
blocks through the journal at all.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>