NFSv4.2 server replies to Copy with length == 0

[Rick Macklem <rmacklem@xxxxxxxxxxx>]

During interop testing (FreeBSD client->Linux server) of NFSv4.2, my
client got into a loop. It was because the reply to Copy was NFS_OK,
but the length in the reply is 0.
(I'll fix the client to fail for this case so it doesn't loop, but...)

The server is Fedora 30 (5.2.18-200 kernel version).
It you think this might be fixed in a newer kernel or you'd like me to do
something with it to get more info, just let me know.

I've attached a snippet of the packet trace. (If the list strips it off, just email
me and I'll send it to you.)

rick

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