mptcp: add accept_subflow re-check

The re-check of pm->accept_subflow with pm->lock held was missing, this
patch fixed it.
Suggested-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/pm.c

@@ -48,7 +48,7 @@ void mptcp_pm_new_connection(struct mptcp_sock *msk, int server_side)
 bool mptcp_pm_allow_new_subflow(struct mptcp_sock *msk)
 {
 	struct mptcp_pm_data *pm = &msk->pm;
-	int ret;
+	int ret = 0;
 
 	pr_debug("msk=%p subflows=%d max=%d allow=%d", msk, pm->subflows,
 		 pm->subflows_max, READ_ONCE(pm->accept_subflow));
@@ -58,9 +58,11 @@ bool mptcp_pm_allow_new_subflow(struct mptcp_sock *msk)
 		return false;
 
 	spin_lock_bh(&pm->lock);
+	if (READ_ONCE(pm->accept_subflow)) {
 		ret = pm->subflows < pm->subflows_max;
 		if (ret && ++pm->subflows == pm->subflows_max)
 			WRITE_ONCE(pm->accept_subflow, false);
+	}
 	spin_unlock_bh(&pm->lock);
 
 	return ret;