doc: Fix acronym "FEKEK" in ecryptfs

"FEFEK" was incorrectly used as acronym for "File Encryption Key
Encryption Key". This replaces all occurences with "FEKEK".

Signed-off-by: Felix Eckhofer <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/security/keys/ecryptfs.rst

···
 ECryptfs is a stacked filesystem which transparently encrypts and decrypts each
 file using a randomly generated File Encryption Key (FEK).
 
-Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEFEK)
+Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEKEK)
 either in kernel space or in user space with a daemon called 'ecryptfsd'.  In
 the former case the operation is performed directly by the kernel CryptoAPI
-using a key, the FEFEK, derived from a user prompted passphrase;  in the latter
+using a key, the FEKEK, derived from a user prompted passphrase;  in the latter
 the FEK is encrypted by 'ecryptfsd' with the help of external libraries in order
 to support other mechanisms like public key cryptography, PKCS#11 and TPM based
 operations.
···
 The 'encrypted' key type has been extended with the introduction of the new
 format 'ecryptfs' in order to be used in conjunction with the eCryptfs
 filesystem.  Encrypted keys of the newly introduced format store an
-authentication token in its payload with a FEFEK randomly generated by the
+authentication token in its payload with a FEKEK randomly generated by the
 kernel and protected by the parent master key.
 
 In order to avoid known-plaintext attacks, the datablob obtained through
 commands 'keyctl print' or 'keyctl pipe' does not contain the overall
-authentication token, which content is well known, but only the FEFEK in
+authentication token, which content is well known, but only the FEKEK in
 encrypted form.
 
 The eCryptfs filesystem may really benefit from using encrypted keys in that the