logfs: destroy the reserved inodes while unmounting

We were assuming that the evict_inode() would never be called on
reserved inodes. However, (after the commit 8e22c1a4e logfs: get rid
of magical inodes) while unmounting the file system, in put_super, we
call iput() on all of the reserved inodes.

The following simple test used to cause a kernel panic on LogFS:

1. Mount a LogFS file system on /mnt

2. Create a file
   $ touch /mnt/a

3. Try to unmount the FS
   $ umount /mnt

The simple fix would be to drop the assumption and properly destroy
the reserved inodes.

Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>

fs/logfs/inode.c

@@ -156,10 +156,26 @@ static void __logfs_destroy_inode(struct inode *inode)
 	call_rcu(&inode->i_rcu, logfs_i_callback);
 }
 
+static void __logfs_destroy_meta_inode(struct inode *inode)
+{
+	struct logfs_inode *li = logfs_inode(inode);
+	BUG_ON(li->li_block);
+	call_rcu(&inode->i_rcu, logfs_i_callback);
+}
+
 static void logfs_destroy_inode(struct inode *inode)
 {
 	struct logfs_inode *li = logfs_inode(inode);
 
+	if (inode->i_ino < LOGFS_RESERVED_INOS) {
+		/*
+		 * The reserved inodes are never destroyed unless we are in
+		 * unmont path.
+		 */
+		__logfs_destroy_meta_inode(inode);
+		return;
+	}
+
 	BUG_ON(list_empty(&li->li_freeing_list));
 	spin_lock(&logfs_inode_lock);
 	li->li_refcount--;