NETIF_F_IP|IPV6_CSUM feature flag indicates UDP and TCP csum offload
while NETIF_F_HW_CSUM feature flag indicates ip generic csum offload
for HW, which includes not only for TCP/UDP csum, but also for other
protocols' csum like GRE's.

However, in skb_csum_hwoffload_help() it only checks features against
NETIF_F_CSUM_MASK(NETIF_F_HW|IP|IPV6_CSUM). So if it's a non TCP/UDP
packet and the features doesn't support NETIF_F_HW_CSUM, but supports
NETIF_F_IP|IPV6_CSUM only, it would still return 0 and leave the HW
to do csum.

This patch is to support ip generic csum processing by checking
NETIF_F_HW_CSUM for all protocols, and check (NETIF_F_IP_CSUM |
NETIF_F_IPV6_CSUM) only for TCP and UDP.

Note that we're using skb->csum_offset to check if it's a TCP/UDP
proctol, this might be fragile. However, as Alex said, for now we
only have a few L4 protocols that are requesting Tx csum offload,
we'd better fix this until a new protocol comes with a same csum
offset.

v1->v2:
  - not extend skb->csum_not_inet, but use skb->csum_offset to tell
    if it's an UDP/TCP csum packet.
v2->v3:
  - add a note in the changelog, as Willem suggested.
Suggested-by: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>