ecryptfs: messaging: Add missing param descriptions and demote abuses

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

 fs/ecryptfs/messaging.c:15: warning: Function parameter or member 'ecryptfs_msg_ctx_free_list' not described in 'LIST_HEAD'
 fs/ecryptfs/messaging.c:15: warning: expecting prototype for eCryptfs(). Prototype was for LIST_HEAD() instead
 fs/ecryptfs/messaging.c:157: warning: Function parameter or member 'daemon' not described in 'ecryptfs_exorcise_daemon'
 fs/ecryptfs/messaging.c:207: warning: Function parameter or member 'daemon' not described in 'ecryptfs_process_response'
 fs/ecryptfs/messaging.c:207: warning: expecting prototype for ecryptfs_process_reponse(). Prototype was for ecryptfs_process_response() instead
 fs/ecryptfs/messaging.c:262: warning: Function parameter or member 'msg_type' not described in 'ecryptfs_send_message_locked'

Cc: Tyler Hicks <code@tyhicks.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Michael A. Halcrow" <mhalcrow@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>