net/mlx5: fix GRE flow item matching

net/mlx5: fix GRE flow item matching

GRE protocol type is implicitly set in the matching translation in case
an application doesn't specify any type explicitly in a flow rule.
It is extracted from the inner header type, but this type may be absent.
In this case, GRE item matching is broken. Check if we have the inner
header type before setting it to allow matching on all GRE packets.

Fixes: be26e81bfc1c ("net/mlx5: fix GRE protocol type translation")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>