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

In the Linux kernel, the following vulnerability has been resolved:

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.