app/testpmd: fix raw encap of GENEVE option

app/testpmd: fix raw encap of GENEVE option

The structure "rte_flow_item_geneve_opt" is not a protocol header of
geneve tunnel option from rfc8926. The field "data" is a pointer
which points to the actual variable-length option data. So the
structure is not packed.

There is 4 bytes hole before the pointer in a 64-bit system. The
option header is just 4 bytes. When using offsetof() to get the
fixed part's size of option header, the wrong value 8 was got. When
constructing the encap header, a wrong size and offset was used due
to this hole.

With this commit, the fixed part's size is calculated explicitly
based on all fields.

Fixes: 55c074f3ba1d ("app/testpmd: support GENEVE option item")
Cc: stable@dpdk.org
Signed-off-by: Bing Zhao <bingz@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>