mctp: fix use after free

Clang static analysis reports this problem
route.c:425:4: warning: Use of memory after it is freed
  trace_mctp_key_acquire(key);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~
When mctp_key_add() fails, key is freed but then is later
used in trace_mctp_key_acquire().  Add an else statement
to use the key only when mctp_key_add() is successful.

Fixes: 4f9e1ba6de ("mctp: Add tracepoints for tag/key handling")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mctp/route.c

@@ -412,13 +412,14 @@ static int mctp_route_input(struct mctp_route *route, struct sk_buff *skb)
 			 * this function.
 			 */
 			rc = mctp_key_add(key, msk);
-			if (rc)
+			if (rc) {
 				kfree(key);
+			} else {
+				trace_mctp_key_acquire(key);
 
-			trace_mctp_key_acquire(key);
-
-			/* we don't need to release key->lock on exit */
-			mctp_key_unref(key);
+				/* we don't need to release key->lock on exit */
+				mctp_key_unref(key);
+			}
 			key = NULL;
 
 		} else {