igc: remove napi_synchronize() in igc_down()

In the Linux kernel, the following vulnerability has been resolved: igc: remove napi_synchronize() in igc_down() When an AF_XDP zero-copy application is killed abruptly, the XSK pool is torn down but NAPI keeps polling. igc_clean_rx_irq_zc() then returns the full budget on every poll, so napi_complete_done() never clears NAPI_STATE_SCHED. igc_down() calls napi_synchronize() before napi_disable(), so it spins forever waiting for that bit and the interface never goes down. Drop the napi_synchronize() and let napi_disable() do the job -- it sets NAPI_STATE_DISABLE, which forces the stuck poll to complete. Reorder it ahead of igc_set_queue_napi() so the NAPI mapping is cleared only after polling has stopped, matching the recent igb fix b1e067240379.