ecryptfs: kthread: Demote file header and provide description for 'cred'

Fixes the following W=1 kernel build warning(s):

 fs/ecryptfs/kthread.c:16: warning: cannot understand function prototype: 'struct ecryptfs_open_req '
 fs/ecryptfs/kthread.c:120: warning: Function parameter or member 'cred' not described in 'ecryptfs_privileged_open'

Cc: Tyler Hicks <code@tyhicks.com>
Cc: "Michael A. Halcrow" <mahalcro@us.ibm.com>
Cc: ecryptfs@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tyler Hicks <code@tyhicks.com>

fs/ecryptfs/kthread.c

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
-/**
+/*
  * eCryptfs: Linux filesystem encryption layer
  *
  * Copyright (C) 2008 International Business Machines Corp.
@@ -108,6 +108,7 @@ void ecryptfs_destroy_kthread(void)
  * @lower_file: Result of dentry_open by root on lower dentry
  * @lower_dentry: Lower dentry for file to open
  * @lower_mnt: Lower vfsmount for file to open
+ * @cred: credential to use for this call
  *
  * This function gets a r/w file opened against the lower dentry.
  *