Keep a separate field, sk_datalen, that tracks only the data contained
in a fragment, not including the fragment header.
For now, this is always just max(0, sk_tcplen - 4), but after we allow
multiple fragments sk_datalen will accumulate the total rpc data size
while sk_tcplen only tracks progress receiving the current fragment.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
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