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one-byte TCP writes wedging

Balraj noticed that a stream of 1-byte writes on a TCP connection would cause 
netfront/netback to wedge.  This is obviously quite unrealistic, but a good 
regression test.

A quick chat with Steven Smith pointed out that some Linux netbacks had a limit 
on the number of fragments allowed (based on the skbuff chain limit size).  So 
you might be ending up with a situation where the backend drops the entire set 
of fragments, and the frontend is retransmitting all the time.

So if you modify our frontend to limit the fragment size to ~10 or so for any 
given packet, that might help.  On the other hand, if you're doing writes with 
a TCP segment size of 1, but still only 3-4 fragments (for the Ethernet/IP/TCP 
headers), then we have some other bug.  What does the Netif request look like, 




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