mptcp: fix bogus receive window shrinkage with multiple subflows

In case multiple subflows race to update the mptcp-level receive
window, the subflow losing the race should use the window value
provided by the "winning" subflow to update it's own tcp-level
rcv_wnd.

To such goal, the current code bogusly uses the mptcp-level rcv_wnd
value as observed before the update attempt. On unlucky circumstances
that may lead to TCP-level window shrinkage, and stall the other end.

Address the issue feeding to the rcv wnd update the correct value.

Fixes: f3589be ("mptcp: never shrink offered window")
Closes: #427
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>

net/mptcp/options.c

@@ -1269,12 +1269,13 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 
 			if (rcv_wnd == rcv_wnd_old)
 				break;
-			if (before64(rcv_wnd_new, rcv_wnd)) {
+
+			rcv_wnd_old = rcv_wnd;
+			if (before64(rcv_wnd_new, rcv_wnd_old)) {
 				MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDCONFLICTUPDATE);
 				goto raise_win;
 			}
 			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDCONFLICT);
-			rcv_wnd_old = rcv_wnd;
 		}
 		return;
 	}