Downgrade "Unexpected onionskin length after decryption" warning

It's now a protocol-warn, since there's nothing relay operators can
do about a client that sends them a malformed create cell.

Resolves bug 12996; bugfix on 0.0.6rc1.

changes/bug12996

@@ -0,0 +1,5 @@
+  o Minor bugfixes:
+    - Downgrade "Unexpected onionskin length after decryption" warning
+      to a protocol-warn, since there's nothing relay operators can do
+      about a client that sends them a malformed create cell. Resolves
+      bug 12996; bugfix on 0.0.6rc1.

src/or/onion_tap.c

@@ -122,8 +122,9 @@ onion_skin_TAP_server_handshake(
              "Couldn't decrypt onionskin: client may be using old onion key");
     goto err;
   } else if (len != DH_KEY_LEN) {
-    log_warn(LD_PROTOCOL, "Unexpected onionskin length after decryption: %ld",
-             (long)len);
+    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
+           "Unexpected onionskin length after decryption: %ld",
+           (long)len);
     goto err;
   }