SUNRPC: Allow the client to detect if the TCP connection is closed

Add an xprt->state bit to enable the TCP ->state_change() method to signal
whether or not the TCP connection is in the process of closing down.
This will to be used by the reconnection logic in a separate patch.

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