cryptfs: Don't hardcode ikey buffer size

We were hardcoding the size of the ikey buffer, but then had logic
which used KEY_LEN_BYTES and IV_LEN_BYTES to offset into the array
and describe the length of its contents.

In anticipation of allowing the keysize to be set via a property,
instead of at compile time, we change this code to make the relation
between the keysize and the buffer size explicit.

Bug: 73079191
Test: Flashed an encrypted sailfish and it booted.
Change-Id: I109a5dc812662220e53163bfb4b5e51bf5abf185

cryptfs.cpp

@@ -1109,7 +1109,7 @@ static int encrypt_master_key(const char *passwd, const unsigned char *salt,
                               unsigned char *encrypted_master_key,
                               struct crypt_mnt_ftr *crypt_ftr)
 {
-    unsigned char ikey[32+32] = { 0 }; /* Big enough to hold a 256 bit key and 256 bit IV */
+    unsigned char ikey[KEY_LEN_BYTES+IV_LEN_BYTES] = { 0 };
     EVP_CIPHER_CTX e_ctx;
     int encrypted_len, final_len;
     int rc = 0;
@@ -1196,7 +1196,7 @@ static int decrypt_master_key_aux(const char *passwd, unsigned char *salt,
                                   unsigned char** intermediate_key,
                                   size_t* intermediate_key_size)
 {
-  unsigned char ikey[32+32] = { 0 }; /* Big enough to hold a 256 bit key and 256 bit IV */
+  unsigned char ikey[KEY_LEN_BYTES+IV_LEN_BYTES] = { 0 };
   EVP_CIPHER_CTX d_ctx;
   int decrypted_len, final_len;