ext4: no need flush workqueue before destroying it

destroy_workqueue() will do flushing work for us.
Signed-off-by: NJun Piao <piaojun@huawei.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Reviewed-by: NJan Kara <jack@suse.cz>

fs/ext4/super.c

@@ -871,7 +871,6 @@ static void ext4_put_super(struct super_block *sb)
 	ext4_unregister_li_request(sb);
 	ext4_quota_off_umount(sb);
 
-	flush_workqueue(sbi->rsv_conversion_wq);
 	destroy_workqueue(sbi->rsv_conversion_wq);
 
 	if (sbi->s_journal) {