net: Eliminate NETIF_F_GEN_CSUM and NETIF_F_V[46]_CSUM

These netif flags are unnecessary convolutions. It is more
straightforward to just use NETIF_F_HW_CSUM, NETIF_F_IP_CSUM,
and NETIF_F_IPV6_CSUM directly.

This patch also:
    - Cleans up can_checksum_protocol
    - Simplifies netdev_intersect_features

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index f938616..1300426 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -60,8 +60,9 @@
 
 	/* Try to offload checksum if possible */
 	offload_csum = !!(need_csum &&
-			  (skb->dev->features &
-			   (is_ipv6 ? NETIF_F_V6_CSUM : NETIF_F_V4_CSUM)));
+			  ((skb->dev->features & NETIF_F_HW_CSUM) ||
+			   (skb->dev->features & (is_ipv6 ?
+			    NETIF_F_IPV6_CSUM : NETIF_F_IP_CSUM))));
 
 	/* segment inner packet. */
 	enc_features = skb->dev->hw_enc_features & features;