fscrypt: constify struct fscrypt_hkdf parameter to fscrypt_hkdf_expand()

Constify the struct fscrypt_hkdf parameter to fscrypt_hkdf_expand().
This makes it clearer that struct fscrypt_hkdf contains the key only,
not any per-request state.

Link: https://lore.kernel.org/r/20191209204054.227736-1-ebiggers@kernel.orgSigned-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/fscrypt_private.h

@@ -287,7 +287,7 @@ extern int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 #define HKDF_CONTEXT_DIRECT_KEY		3
 #define HKDF_CONTEXT_IV_INO_LBLK_64_KEY	4
 
-extern int fscrypt_hkdf_expand(struct fscrypt_hkdf *hkdf, u8 context,
+extern int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
 			       const u8 *info, unsigned int infolen,
 			       u8 *okm, unsigned int okmlen);
 

fs/crypto/hkdf.c

@@ -112,7 +112,7 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
  * adds to its application-specific info strings to guarantee that it doesn't
  * accidentally repeat an info string when using HKDF for different purposes.)
  */
-int fscrypt_hkdf_expand(struct fscrypt_hkdf *hkdf, u8 context,
+int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
 			const u8 *info, unsigned int infolen,
 			u8 *okm, unsigned int okmlen)
 {