When the guest sends data on a TCP connection, it's possible that we run
out of buffer space on the socket side. If that happens we'll advertise
a zero window to the guest stopping it from sending. When the socket side
buffer clears again, which is signalled by an EPOLLOUT, we recalculate the
ack and window with tcp_update_seqack_wnd().
tcp_update_seqack_wnd() only calculates the new values, it doesn't actually
send out the ack - that will typically happen some milliseconds later on
the ACK_TO_TAP_DUE timer.
AFAICT, there's not really any point delaying this ack though, we might as
well send it and get the guest sending again ASAP. So, instead of
calling tcp_update_seqack_wnd() call tcp_send_flag() to send an immediate
ACK if needed.
Signed-off-by: David Gibson