SUNRPC: Use shutdown() instead of close() when disconnecting a TCP socket

By using shutdown() rather than close() we allow the RPC client to wait
for the TCP close handshake to complete before we start trying to reconnect
using the same port.
We use shutdown(SHUT_WR) only instead of shutting down both directions,
however we wait until the server has closed the connection on its side.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>