SUNRPC: Ensure we set XPRT_CLOSING only after we've sent a tcp FIN...

...so that we can distinguish between when we need to shutdown and when we
don't. Also remove the call to xs_tcp_shutdown() from xs_tcp_connect(),
since xprt_connect() makes the same test.

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