SUNRPC: Add API to force the client to disconnect

Allow the caller to force a disconnection of the RPC client so that we
can clear any pending requests that are buffered in the socket.

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