mptcp: export mptcp_subflow_early_fallback()

This helper will be used outside protocol.h in the following commit.

While at it, also add a 'pr_fallback()' debug print, to help identifying
fallbacks.
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240909-net-next-mptcp-fallback-x-mpc-v1-1-da7ebb4cd2a3@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -3717,13 +3717,6 @@ static int mptcp_ioctl(struct sock *sk, int cmd, int *karg)
 	return 0;
 }
 
-static void mptcp_subflow_early_fallback(struct mptcp_sock *msk,
-					 struct mptcp_subflow_context *subflow)
-{
-	subflow->request_mptcp = 0;
-	__mptcp_do_fallback(msk);
-}
-
 static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 {
 	struct mptcp_subflow_context *subflow;

net/mptcp/protocol.h

@@ -1215,6 +1215,14 @@ static inline void mptcp_do_fallback(struct sock *ssk)
 
 #define pr_fallback(a) pr_debug("%s:fallback to TCP (msk=%p)\n", __func__, a)
 
+static inline void mptcp_subflow_early_fallback(struct mptcp_sock *msk,
+						struct mptcp_subflow_context *subflow)
+{
+	pr_fallback(msk);
+	subflow->request_mptcp = 0;
+	__mptcp_do_fallback(msk);
+}
+
 static inline bool mptcp_check_infinite_map(struct sk_buff *skb)
 {
 	struct mptcp_ext *mpext;