ecryptfs: keystore: Fix some kernel-doc issues and demote non-conformant headers

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

 fs/ecryptfs/keystore.c:25: warning: Incorrect use of kernel-doc format:  * request_key returned an error instead of a valid key address;
 fs/ecryptfs/keystore.c:30: warning: Function parameter or member 'err_code' not described in 'process_request_key_err'
 fs/ecryptfs/keystore.c:30: warning: expecting prototype for eCryptfs(). Prototype was for process_request_key_err() instead
 fs/ecryptfs/keystore.c:558: warning: Function parameter or member 'auth_tok_key' not described in 'ecryptfs_find_auth_tok_for_sig'
 fs/ecryptfs/keystore.c:558: warning: Function parameter or member 'mount_crypt_stat' not described in 'ecryptfs_find_auth_tok_for_sig'
 fs/ecryptfs/keystore.c:558: warning: Excess function parameter 'crypt_stat' description in 'ecryptfs_find_auth_tok_for_sig'
 fs/ecryptfs/keystore.c:584: warning: cannot understand function prototype: 'struct ecryptfs_write_tag_70_packet_...