fscrypt: refactor key setup code in preparation for v2 policies

Do some more refactoring of the key setup code, in preparation for
introducing a filesystem-level keyring and v2 encryption policies:

- Now that ci_inode exists, don't pass around the inode unnecessarily.

- Define a function setup_file_encryption_key() which handles the crypto
  key setup given an under-construction fscrypt_info.  Don't pass the
  fscrypt_context, since everything is in the fscrypt_info.
  [This will be extended for v2 policies and the fs-level keyring.]

- Define a function fscrypt_set_derived_key() which sets the per-file
  key, without depending on anything specific to v1 policies.
  [This will also be used for v2 policies.]

- Define a function fscrypt_setup_v1_file_key() which takes the raw
  master key, thus separating finding the key from using it.
  [This will also be used if the key is found in the fs-level keyring.]
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/fscrypt_private.h

@@ -4,9 +4,8 @@
  *
  * Copyright (C) 2015, Google, Inc.
  *
- * This contains encryption key functions.
- *
- * Written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar, 2015.
+ * Originally written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar.
+ * Heavily modified since then.
  */
 
 #ifndef _FSCRYPT_PRIVATE_H
@@ -168,4 +167,10 @@ struct fscrypt_mode {
 	bool needs_essiv;
 };
 
+static inline bool
+fscrypt_mode_supports_direct_key(const struct fscrypt_mode *mode)
+{
+	return mode->ivsize >= offsetofend(union fscrypt_iv, nonce);
+}
+
 #endif /* _FSCRYPT_PRIVATE_H */