sfc: Don't invoke xdp_do_flush() from netpoll.

Yury reported a crash in the sfc driver originated from
netpoll_send_udp(). The netconsole sends a message and then netpoll
invokes the driver's NAPI function with a budget of zero. It is
dedicated to allow driver to free TX resources, that it may have used
while sending the packet.

In the netpoll case the driver invokes xdp_do_flush() unconditionally,
leading to crash because bpf_net_context was never assigned.

Invoke xdp_do_flush() only if budget is not zero.

Fixes: 401cb7d ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Reported-by: Yury Vostrikov <[email protected]>
Closes: https://lore.kernel.org/[email protected]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Reviewed-by: Edward Cree <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Sebastian Andrzej Siewior

drivers/net/ethernet/sfc/efx_channels.c

@@ -1260,7 +1260,8 @@ static int efx_poll(struct napi_struct *napi, int budget)
 
 	spent = efx_process_channel(channel, budget);
 
-	xdp_do_flush();
+	if (budget)
+		xdp_do_flush();
 
 	if (spent < budget) {
 		if (efx_channel_has_rx_queue(channel) &&

drivers/net/ethernet/sfc/siena/efx_channels.c

@@ -1285,7 +1285,8 @@ static int efx_poll(struct napi_struct *napi, int budget)
 
 	spent = efx_process_channel(channel, budget);
 
-	xdp_do_flush();
+	if (budget)
+		xdp_do_flush();
 
 	if (spent < budget) {
 		if (efx_channel_has_rx_queue(channel) &&