net: mana: report RX checksum offload for IPv6

The hardware validates TCP and UDP checksums for both IPv4 and IPv6 and
reports the result in rx_tcp_csum_succeed and rx_udp_csum_succeed.
However, mana_rx_skb() also requires rx_iphdr_csum_succeed before it
sets CHECKSUM_UNNECESSARY. That bit is only set for IPv4 (since IPv6
has no header checksum), so IPv6 TCP and UDP traffic is always
checksummed again in software.

So, only look at the L4 result. For IPv4 this makes no material
difference since CHECKSUM_UNNECESSARY only covers the L4 checksum, and
ip_rcv_core() always verifies the IPv4 header checksum in software.

Between two Azure Standard_E16s_v6 VMs (VF enabled), IPv6 TCP and UDP
packets (including VXLAN over IPv6), now arrive with
CHECKSUM_UNNECESSARY instead of CHECKSUM_NONE, so GRO no longer
checksums them in software. Calls to __skb_gro_checksum_complete() on
the receiver during an 8 second iperf3 run:

                               before    after
  TCP/IPv4, 12.2 Gbps              23       28
  TCP/IPv6, 12.1 Gbps         8728419       25
  UDP/IPv6, 1.5 Gbps          1071380       20
  TCP/VXLAN/IPv6, 10.9 Gbps   7934373       18

Cc: stable@vger.kernel.org
Fixes: ca9c54d2d6a5 ("net: mana: Add a driver for Microsoft Azure Network Adapter (MANA)")
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
Link: https://patch.msgid.link/20261109142856.779955-1-hamzamahfooz@linux.microsoft.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>