mptcp: use list_first_entry_or_null

Use list_first_entry_or_null to simplify the code.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.h

@@ -232,10 +232,7 @@ static inline struct mptcp_data_frag *mptcp_rtx_head(const struct sock *sk)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
 
-	if (list_empty(&msk->rtx_queue))
-		return NULL;
-
-	return list_first_entry(&msk->rtx_queue, struct mptcp_data_frag, list);
+	return list_first_entry_or_null(&msk->rtx_queue, struct mptcp_data_frag, list);
 }
 
 struct mptcp_subflow_request_sock {