tcp: enable mid stream window clamp

The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
of the advertised window to this value."  Window clamping is distributed
across two variables, window_clamp ("Maximal window to advertise" in
tcp.h) and rcv_ssthresh ("Current window clamp").

This patch updates the function where the window clamp is set to also
reduce the current window clamp, rcv_sshthresh, if needed.  With this,
setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
the window.

Signed-off-by: Neil Spring <ntspring@fb.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20210825210117.1668371-1-ntspring@fb.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>