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/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index e6a7bd15..2f74845 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1322,7 +1322,7 @@
 	    headersize == sizeof(struct ipv6hdr) &&
 	    length < mtu - headersize &&
 	    !(flags & MSG_MORE) &&
-	    rt->dst.dev->features & NETIF_F_V6_CSUM)
+	    rt->dst.dev->features & (NETIF_F_IPV6_CSUM | NETIF_F_HW_CSUM))
 		csummode = CHECKSUM_PARTIAL;
 
 	if (sk->sk_type == SOCK_DGRAM || sk->sk_type == SOCK_RAW) {