fscrypt: rename FS_KEY_DERIVATION_NONCE_SIZE

The name "FS_KEY_DERIVATION_NONCE_SIZE" is a bit outdated since due to
the addition of FSCRYPT_POLICY_FLAG_DIRECT_KEY, the file nonce may now
be used as a tweak instead of for key derivation.  Also, we're now
prefixing the fscrypt constants with "FSCRYPT_" instead of "FS_".

Therefore, rename this constant to FSCRYPT_FILE_NONCE_SIZE.

Link: https://lore.kernel.org/r/20200708215722.147154-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/policy.c

@@ -529,7 +529,7 @@ int fscrypt_ioctl_get_nonce(struct file *filp, void __user *arg)
 	if (!fscrypt_context_is_valid(&ctx, ret))
 		return -EINVAL;
 	if (copy_to_user(arg, fscrypt_context_nonce(&ctx),
-			 FS_KEY_DERIVATION_NONCE_SIZE))
+			 FSCRYPT_FILE_NONCE_SIZE))
 		return -EFAULT;
 	return 0;
 }