[LogFS] Only write journal if dirty

This prevents unnecessary journal writes.  More importantly it prevents
an oops due to a journal write on failed mount.
diff --git a/fs/logfs/super.c b/fs/logfs/super.c
index 94d80f7..1d081b7 100644
--- a/fs/logfs/super.c
+++ b/fs/logfs/super.c
@@ -490,7 +490,7 @@
 	log_super("LogFS: Start unmounting\n");
 	/* Alias entries slow down mount, so evict as many as possible */
 	sync_filesystem(sb);
-	logfs_write_anchor(super->s_master_inode);
+	logfs_write_anchor(sb);
 
 	/*
 	 * From this point on alias entries are simply dropped - and any