Superblock of raid5 log can't be updated when array stoped

[liuzhengyuan <liuzhengyuan@kylinos.cn>]

Hi, Shaohua.

when we stop raid5 array with "mdadm -S" or reboot the system,  md module will 
call raid5_quiesce and r5l_quiesce to do some clean work. Some code of r5l_quiesce
was pasted bellow.

                /* make sure r5l_write_super_and_discard_space exits */
                mddev = log->rdev->mddev;
                wake_up(&mddev->sb_wait);
                r5l_wake_reclaim(log, -1L);
                md_unregister_thread(&log->reclaim_thread);
                r5l_do_reclaim(log);
+                md_update_sb(mddev, 1);

It will reclaim all used space of log and call r5l_write_super to reset rdev->journal_tail
to log->next_checkpoint . However, new rdev->journal_tail would not be written to 
journal device for persistent because journal device may not support discard operation
or due to mddev_trylock fail (this trylock should always get failed since raid5_quiesce 
was called with  reconfig_mutex hold, isn't it?). As a result, it will take a long time to
 recovery the log when the arrary was restarted. Should r5l_quiesce call md_update_sb
 directly to guarantee  superblock  update?