SUNRPC: Ensure the transport backchannel association

If the server sends CB_ calls on a connection that is not associated
with the backchannel, refuse to process the call and shut down the
connection.  This avoids a NULL dereference crash in
xprt_complete_bc_request().  There's not much more we can do in this
situation unless we want to look into allowing all connections to be
associated with the fore and back channel.
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xprtsock.c

@@ -558,6 +558,10 @@ xs_read_stream_call(struct sock_xprt *transport, struct msghdr *msg, int flags)
 	struct rpc_rqst *req;
 	ssize_t ret;
 
+	/* Is this transport associated with the backchannel? */
+	if (!xprt->bc_serv)
+		return -ESHUTDOWN;
+
 	/* Look up and lock the request corresponding to the given XID */
 	req = xprt_lookup_bc_request(xprt, transport->recv.xid);
 	if (!req) {