ecryptfs: crypto: Supply some missing param descriptions and demote abuses

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

 fs/ecryptfs/crypto.c:29: warning: expecting prototype for eCryptfs(). Prototype was for DECRYPT() instead
 fs/ecryptfs/crypto.c:360: warning: Function parameter or member 'crypt_stat' not described in 'lower_offset_for_page'
 fs/ecryptfs/crypto.c:360: warning: Function parameter or member 'page' not described in 'lower_offset_for_page'
 fs/ecryptfs/crypto.c:637: warning: Function parameter or member 'crypt_stat' not described in 'ecryptfs_compute_root_iv'
 fs/ecryptfs/crypto.c:1386: warning: Function parameter or member 'ecryptfs_dentry' not described in 'ecryptfs_read_metadata'
 fs/ecryptfs/crypto.c:1463: warning: Function parameter or member 'filename' not described in 'ecryptfs_encrypt_filename'
 fs/ecryptfs/crypto.c:1463: warning: Function parameter or member 'mount_crypt_stat' not described in 'ecryptfs_encrypt_filename'
 fs/ecryptfs/crypto.c:1897: warning: Function parameter or member 'encoded_name_size' not described in 'ecryptfs_encrypt_and_encode_filename'
 fs/ecryptfs/crypto.c:1897: warning: Function parameter or member 'mount_crypt_stat' not described in 'ecryptfs_encrypt_and_encode_filename'
 fs/ecryptfs/crypto.c:1897: warning: Function parameter or member 'name_size' not described in 'ecryptfs_encrypt_and_encode_filename'
 fs/ecryptfs/crypto.c:1897: warning: Excess function parameter 'crypt_stat' description in 'ecryptfs_encrypt_and_encode_filename'
 fs/ecryptfs/crypto.c:1897: warning: Excess function parameter 'length' description in 'ecryptfs_encrypt_and_encode_filename'
 fs/ecryptfs/crypto.c:2006: warning: Function parameter or member 'sb' not described in 'ecryptfs_decode_and_decrypt_filename'
 fs/ecryptfs/crypto.c:2006: warning: Excess function parameter 'ecryptfs_dir_dentry' description in 'ecryptfs_decode_and_decrypt_filename'

Cc: Tyler Hicks <code@tyhicks.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: "Michael A. Halcrow" <mahalcro@us.ibm.com>
Cc: "Michael C. Thompson" <mcthomps@us.ibm.com>
Cc: ecryptfs@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
[tyhicks: Fix typo in ecryptfs_encrypt_and_encode_filename() func docs]
Signed-off-by: Tyler Hicks <code@tyhicks.com>