Prevent a flurry of forced window updates when an application is doing small reads on a (partially) filled receive socket buffer.
Prevent a flurry of forced window updates when an application is
doing small reads on a (partially) filled receive socket buffer.

Normally one would a send a window update every time the available
space in the socket buffer increases by two times MSS.  This leads
to a flurry of window updates that do not provide any meaningful
new information to the sender.  There still is available space in
the window and the sender can continue sending data.  All window
updates then get carried by the regular ACKs.  Only when the socket
buffer was (almost) full and the window closed accordingly a window
updates delivery new information and allows the sender to start
sending more data again.

Send window updates only every two MSS when the socket buffer
has less than 1/8 space available, or the available space in the
socket buffer increased by 1/4 its full capacity, or the socket
buffer is very small.  The next regular data ACK will carry and
report the exact window size again.

Reported by:	sbruno
Tested by:	darrenr
Tested by:	Darren Baginski
PR:		kern/116335
MFC after:	2 weeks

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