sctp: switch sctp_setsockopt_auth_key to use memzero_explicit

Switch from kzfree to sctp_setsockopt_auth_key + kfree to prepare for
moving the kfree to common code.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -3649,7 +3649,8 @@ static int sctp_setsockopt_auth_key(struct sock *sk,
 	}
 
 out:
-	kzfree(authkey);
+	memzero_explicit(authkey, optlen);
+	kfree(authkey);
 	return ret;
 }