app/testpmd: fix GENEVE flow item

app/testpmd: fix GENEVE flow item

Previous patch added support of GENEVE pattern item in flow rule.
Function flow_item_default_mask() was not updated, so using it with
GENEVE item returns null.
Using testpmd command "set raw_decap" or "set raw_encap" with
GENEVE item, without specifying any parameters, results in
segmentation fault.

This patch updates function flow_item_default_mask(),
adding case to handle GENEVE item.

Fixes: 0f4203fe9d18 ("app/testpmd: support GENEVE pattern item in flow rules")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>