SUNRPC: Tweak TCP socket shutdown in the RPC client

We only really need to call shutdown() if we're in the ESTABLISHED TCP
state, since that is the only case where the client is initiating a
close of an established connection.

If the socket is in FIN_WAIT1 or FIN_WAIT2, then we've already initiated
socket shutdown and are waiting for the server's reply, so do nothing.

In all other cases where we've already received a FIN from the server,
we should be able to just close the socket.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>