net/mlx5: fix VXLAN-GPE next protocol translation

net/mlx5: fix VXLAN-GPE next protocol translation

VXLAN-GPE extends VXLAN protocol and provides the next protocol
field specifying the first inner header type.

The application can assign some explicit value to
VXLAN-GPE::next_protocol field or set it to the default one. In the
latter case, the rdma-core library cannot recognize the matcher
built by PMD correctly, and it results in hardware configuration
missing inner headers match.

The patch forces VXLAN-GPE::next_protocol assignment if the
application did not explicitly assign it to the non-default value

Fixes: 90456726eb80 ("net/mlx5: fix VXLAN-GPE item translation")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>