net/mlx5: fix setting offsets for SW parser

net/mlx5: fix setting offsets for SW parser

Since ConnectX-5, SW parser just complements HW parser. SW parser starts to
engage only if HW parser can't reach a header. For the older devices, HW
parser will not kick in if any of SWP offsets is set. Therefore, all of the
L3 offsets should be set regardless of HW offload. As IPv6 doesn't have
header checksum, the mbuf can't have PKT_TX_[OUTER_]IP_CKSUM if outer or
inner L3 is IPv6.

And if inner packet isn't IP, the inner offsets shouldn't be set.

Fixes: 5f8ba81c4228 ("net/mlx5: support generic tunnel offloading")

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Xueming Li <xuemingl@mellanox.com>