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>