From: Angus Sawyer <angus.sawyer@dsl.pipex.com>

If there are no writes for 20 milliseconds, write out superblock
to mark array as clean.  Write out superblock with
dirty flag before allowing any further write to succeed.

If an md thread gets signaled with SIGKILL, reduce the
delay to 0.


Also tidy up some printk's and make sure writing the
superblock isn't noisy.