net: thunderx: Fix TCP/UDP checksum offload for IPv4 pkts

Offload IP header checksum to NIC.

This fixes a previous patch which disabled checksum offloading
for both IPv4 and IPv6 packets.  So L3 checksum offload was
getting disabled for IPv4 pkts.  And HW is dropping these pkts
for some reason.

Without this patch, IPv4 TSO appears to be broken:

WIthout this patch I get ~16kbyte/s, with patch close to 2mbyte/s
when copying files via scp from test box to my home workstation.

Looking at tcpdump on sender it looks like hardware drops IPv4 TSO skbs.
This patch restores performance for me, ipv6 looks good too.

Fixes: fa6d7cb5 ("net: thunderx: Fix TCP/UDP checksum offload for IPv6 pkts")
Cc: Sunil Goutham <sgoutham@cavium.com>
Cc: Aleksey Makarov <aleksey.makarov@auriga.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>