modify according to the change of keystore interface.

Change-Id: I391dae3fff391f4c67bb34b512f2e9e60182e809
diff --git a/mtpd.c b/mtpd.c
index bf83ac9..fd553be 100644
--- a/mtpd.c
+++ b/mtpd.c
@@ -151,7 +151,7 @@
      * so other files are clean and free from android specific code. */
     if (i > 4 && !strcmp("l2tp", args[1]) && args[4][0]) {
         char value[KEYSTORE_MESSAGE_SIZE];
-        int length = keystore_get(args[4], value);
+        int length = keystore_get(args[4], strlen(args[4]), value);
         if (length == -1) {
             log_print(FATAL, "Cannot get L2TP secret from keystore");
             exit(SYSTEM_ERROR);