SUNRPC: Simplify socket shutdown when not reusing TCP ports

If we're not required to reuse the TCP port, then we can just
immediately close the socket, and leave the cleanup details to the TCP
layer.

Fixes: e6237b6f ("NFSv4.1: Don't rebind to the same source port when reconnecting to the server")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>