SUNRPC: Fix a re-entrancy bug in xs_tcp_read_calldir()

If the attempt to read the calldir fails, then instead of storing the read
bytes, we currently discard them. This leads to a garbage final result when
upon re-entry to the same routine, we read the remaining bytes.

Fixes the regression in bugzilla number 16213. Please see
    https://bugzilla.kernel.org/show_bug.cgi?id=16213



Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org