[TCP]: Fix shrinking windows with window scaling

When selecting a new window, tcp_select_window() tries not to shrink
the offered window by using the maximum of the remaining offered window
size and the newly calculated window size. The newly calculated window
size is always a multiple of the window scaling factor, the remaining
window size however might not be since it depends on rcv_wup/rcv_nxt.
This means we're effectively shrinking the window when scaling it down.


The dump below shows the problem (scaling factor 2^7):

- Window size of 557 (71296) is advertised, up to 3111907257:

IP 172.2.2.3.33000 > 172.2.2.2.33000: . ack 3111835961 win 557 <...>

- New window size of 514 (65792) is advertised, up to 3111907217, 40 bytes
  below the last end:

IP 172.2.2.3.33000 > 172.2.2.2.33000: . 3113575668:3113577116(1448) ack 3111841425 win 514 <...>

The number 40 results from downscaling the remaining window:

3111907257 - 3111841425 = 65832
65832 / 2^7 = 514
65832 % 2^7 = 40

If the sender uses up the entire window before it is shrunk, this can have
chaotic effects on the connection. When sending ACKs, tcp_acceptable_seq()
will notice that the window has been shrunk since tcp_wnd_end() is before
tp->snd_nxt, which makes it choose tcp_wnd_end() as sequence number.
This will fail the receivers checks in tcp_sequence() however since it
is before it's tp->rcv_wup, making it respond with a dupack.

If both sides are in this condition, this leads to a constant flood of
ACKs until the connection times out.

Make sure the window is never shrunk by aligning the remaining window to
the window scaling factor.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>