mptcp: pm: ADD_ADDR rtx: return early if no retrans

No need to iterate over all subflows if there is no retransmission
needed.

Exit early in this case then.

Fixes: 30549ee ("mptcp: make ADD_ADDR retransmission timeout adaptive")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260505-net-mptcp-pm-fixes-7-1-rc3-v1-8-fca8091060a4@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

net/mptcp/pm.c

@@ -308,6 +308,9 @@ static unsigned int mptcp_adjust_add_addr_timeout(struct mptcp_sock *msk)
 	struct mptcp_subflow_context *subflow;
 	unsigned int max = 0, max_stale = 0;
 
+	if (!rto)
+		return 0;
+
 	mptcp_for_each_subflow(msk, subflow) {
 		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
 		struct inet_connection_sock *icsk = inet_csk(ssk);