mctp: test: fix skb free in test device tx

In our test device, we're currently freeing skbs in the transmit path
with kfree(), rather than kfree_skb(). This change uses the correct
kfree_skb() instead.

Fixes: ded21b7229 ("mctp: Add test utils")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mctp/test/utils.c

@@ -12,7 +12,7 @@
 static netdev_tx_t mctp_test_dev_tx(struct sk_buff *skb,
 				    struct net_device *ndev)
 {
-	kfree(skb);
+	kfree_skb(skb);
 	return NETDEV_TX_OK;
 }