SUNRPC: Fix a client regression when handling oversized replies

If the server sends a reply that is larger than the pre-allocated
buffer, then the current code may fail to register how much of
the stream that it has finished reading. This again can lead to
hangs.

Fixes: e92053a5

 ("SUNRPC: Handle zero length fragments correctly")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>