When pre-allocating skbs for received packets, we set ip_summed =
CHECKSUM_UNNCESSARY.  We used to change it back to CHECKSUM_NONE when
the received packet had an incorrect checksum or unhandled protocol.
Commit 
bc8acf2c8c3e43fcc192762a9f964b3e9a17748b ('drivers/net: avoid
some skb->ip_summed initializations') mistakenly replaced the latter
assignment with a DEBUG-only assertion that ip_summed ==
CHECKSUM_NONE.  This assertion is always false, but it seems no-one
has exercised this code path in a DEBUG build.
Fix this by moving our assignment of CHECKSUM_UNNECESSARY into
efx_rx_packet_gro().
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
                if (unlikely(!skb))
                        return -ENOMEM;
 
-               /* Adjust the SKB for padding and checksum */
+               /* Adjust the SKB for padding */
                skb_reserve(skb, NET_IP_ALIGN);
                rx_buf->len = skb_len - NET_IP_ALIGN;
                rx_buf->is_page = false;
-               skb->ip_summed = CHECKSUM_UNNECESSARY;
 
                rx_buf->dma_addr = pci_map_single(efx->pci_dev,
                                                  skb->data, rx_buf->len,
 
                EFX_BUG_ON_PARANOID(!checksummed);
                rx_buf->u.skb = NULL;
+               skb->ip_summed = CHECKSUM_UNNECESSARY;
 
                gro_result = napi_gro_receive(napi, skb);
        }