mptcp: factor out a basic skb coalesce helper

The upcoming patch will introduced backlog processing for MPTCP
socket, and we want to leverage coalescing in such data path.

Factor out the relevant bits not touching memory accounting to
deal with such use-case.

Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250927-net-next-mptcp-rcv-path-imp-v1-6-5da266aa9c1a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

net/mptcp/protocol.c

@@ -142,22 +142,33 @@ static void mptcp_drop(struct sock *sk, struct sk_buff *skb)
 	__kfree_skb(skb);
 }
 
-static bool mptcp_try_coalesce(struct sock *sk, struct sk_buff *to,
-			       struct sk_buff *from)
+static bool __mptcp_try_coalesce(struct sock *sk, struct sk_buff *to,
+				 struct sk_buff *from, bool *fragstolen,
+				 int *delta)
 {
-	bool fragstolen;
-	int delta;
+	int limit = READ_ONCE(sk->sk_rcvbuf);
 
 	if (unlikely(MPTCP_SKB_CB(to)->cant_coalesce) ||
 	    MPTCP_SKB_CB(from)->offset ||
-	    ((to->len + from->len) > (sk->sk_rcvbuf >> 3)) ||
-	    !skb_try_coalesce(to, from, &fragstolen, &delta))
+	    ((to->len + from->len) > (limit >> 3)) ||
+	    !skb_try_coalesce(to, from, fragstolen, delta))
 		return false;
 
 	pr_debug("colesced seq %llx into %llx new len %d new end seq %llx\n",
 		 MPTCP_SKB_CB(from)->map_seq, MPTCP_SKB_CB(to)->map_seq,
 		 to->len, MPTCP_SKB_CB(from)->end_seq);
 	MPTCP_SKB_CB(to)->end_seq = MPTCP_SKB_CB(from)->end_seq;
+	return true;
+}
+
+static bool mptcp_try_coalesce(struct sock *sk, struct sk_buff *to,
+			       struct sk_buff *from)
+{
+	bool fragstolen;
+	int delta;
+
+	if (!__mptcp_try_coalesce(sk, to, from, &fragstolen, &delta))
+		return false;
 
 	/* note the fwd memory can reach a negative value after accounting
 	 * for the delta, but the later skb free will restore a non