mptcp: fix invalid addr occupy 'add_addr_accepted'

This patch fixes an issue where an invalid address is announce as a
signal, the 'add_addr_accepted' is incorrectly added several times
when 'retransmit ADD_ADDR'. So we need to update this variable
when the connection is removed from conn_list by mptcp_worker. So that
the available address can be added in time.

In fact, the 'add_addr_accepted' is only declined when 'RM_ADDR'
by now, so when subflows are getting closed from the other peer,
the new signal is not accepted as well.

We noticed there have exist some problems related to this.I think
this patch effectively resolves them.

Closes: multipath-tcp/mptcp_net-next#498
Signed-off-by: Gang Yan <[email protected]>

net/mptcp/protocol.c

@@ -2575,6 +2575,10 @@ static void __mptcp_close_subflow(struct sock *sk)
 			continue;
 
 		mptcp_close_ssk(sk, ssk, subflow);
+
+		if (READ_ONCE(subflow->remote_id) &&
+		    --msk->pm.add_addr_accepted < mptcp_pm_get_add_addr_accept_max(msk))
+			WRITE_ONCE(msk->pm.accept_addr, true);
 	}
 
 }