debugfs: fix inode i_nlink references for automount dentry

Directory inodes should start off with i_nlink == 2 (one extra ref
for "." entry).  debugfs_create_automount() increases neither the
i_nlink reference for current inode nor for parent inode.

On attempt to remove the automount dentry, kernel complains:

  [   86.288070] WARNING: CPU: 1 PID: 3616 at fs/inode.c:273 drop_nlink+0x3e/0x50()
  [   86.288461] Modules linked in: debugfs_example2(O-)
  [   86.288745] CPU: 1 PID: 3616 Comm: rmmod Tainted: G           O    4.4.0-rc3-next-20151207+ #135
  [   86.289197] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.2-20150617_082717-anatol 04/01/2014
  [   86.289696]  ffffffff81be05c9 ffff8800b9e6fda0 ffffffff81352e2c 0000000000000000
  [   86.290110]  ffff8800b9e6fdd8 ffffffff81065142 ffff8801399175e8 ffff8800bb78b240
  [   86.290507]  ffff8801399175e8 ffff8800b73d7898 ffff8800b73d7840 ffff8800b9e6fde8
  [   86.290933] Call Trace:
  [   86.291080]  [<ffffffff81352e2c>] dump_stack+0x4e/0x82
  [   86.291340]  [<ffffffff81065142>] warn_slowpath_common+0x82/0xc0
  [   86.291640]  [<ffffffff8106523a>] warn_slowpath_null+0x1a/0x20
  [   86.291932]  [<ffffffff811ae62e>] drop_nlink+0x3e/0x50
  [   86.292208]  [<ffffffff811ba35b>] simple_unlink+0x4b/0x60
  [   86.292481]  [<ffffffff811ba3a7>] simple_rmdir+0x37/0x50
  [   86.292748]  [<ffffffff812d9808>] __debugfs_remove.part.16+0xa8/0xd0
  [   86.293082]  [<ffffffff812d9a0b>] debugfs_remove_recursive+0xdb/0x1c0
  [   86.293406]  [<ffffffffa00004dd>] cleanup_module+0x2d/0x3b [debugfs_example2]
  [   86.293762]  [<ffffffff810d959b>] SyS_delete_module+0x16b/0x220
  [   86.294077]  [<ffffffff818ef857>] entry_SYSCALL_64_fastpath+0x12/0x6a
  [   86.294405] ---[ end trace c9fc53353fe14a36 ]---
  [   86.294639] ------------[ cut here ]------------

To reproduce the issue it is enough to invoke these lines:

     autom = debugfs_create_automount("automount", NULL, vfsmount_cb, data);
     BUG_ON(IS_ERR_OR_NULL(autom));
     debugfs_remove(autom);

The issue is fixed by increasing inode i_nlink references for current
and parent inodes.
Signed-off-by: Roman Pen <r.peniaev@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/debugfs/inode.c

@@ -461,7 +461,11 @@ struct dentry *debugfs_create_automount(const char *name,
 	inode->i_flags |= S_AUTOMOUNT;
 	inode->i_private = data;
 	dentry->d_fsdata = (void *)f;
+	/* directory inodes start off with i_nlink == 2 (for "." entry) */
+	inc_nlink(inode);
 	d_instantiate(dentry, inode);
+	inc_nlink(d_inode(dentry->d_parent));
+	fsnotify_mkdir(d_inode(dentry->d_parent), dentry);
 	return end_creating(dentry);
 }
 EXPORT_SYMBOL(debugfs_create_automount);