nfp: fix incorrectly set csum flag for nfdk path

The csum flag of IPsec packet are set repeatedly. Therefore, the csum
flag set of IPsec and non-IPsec packet need to be distinguished.

As the ipv6 header does not have a csum field, so l3-csum flag is not
required to be set for ipv6 case.

Fixes: 436396f2 ("nfp: support IPsec offloading for NFP3800")
Signed-off-by: Huanhuan Wang <huanhuan.wang@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>