net: bcmgenet: skip invalid partial checksums

The RXCHK block will return a partial checksum of 0 if it encounters
a problem while receiving a packet. Since a 1's complement sum can
only produce this result if no bits are set in the received data
stream it is fair to treat it as an invalid partial checksum and
not pass it up the stack.

Fixes: 8101553 ("net: bcmgenet: use CHECKSUM_COMPLETE for NETIF_F_RXCSUM")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220317012812.1313196-1-opendmb@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Doug Berger

drivers/net/ethernet/broadcom/genet/bcmgenet.c

@@ -2287,8 +2287,10 @@ static unsigned int bcmgenet_desc_rx(struct bcmgenet_rx_ring *ring,
 		dma_length_status = status->length_status;
 		if (dev->features & NETIF_F_RXCSUM) {
 			rx_csum = (__force __be16)(status->rx_csum & 0xffff);
-			skb->csum = (__force __wsum)ntohs(rx_csum);
-			skb->ip_summed = CHECKSUM_COMPLETE;
+			if (rx_csum) {
+				skb->csum = (__force __wsum)ntohs(rx_csum);
+				skb->ip_summed = CHECKSUM_COMPLETE;
+			}
 		}
 
 		/* DMA flags and length are still valid no matter how