net/mlx5: fix crash in NVGRE item translation

net/mlx5: fix crash in NVGRE item translation

The flow_dv_translate_item_nvgre function add NVGRE item to matcher and
to the value.
It defines a pointer named nvrge_m that receives the item's mask into
it, and then copies some of it to the matcher.

Before copying, it checks for mask validation, and in case the mask is
NULL the function gives it a pointer to rte_flow_item_nvgre_mask.
However, the function calls from the vni mask's field before the check,
and if there is no mask, it actually does dereference to the NULL
pointer and indeed the program crashes with segfault.

Move the call from the vni field to post-validation.

Fixes: cd18e1b72f73 ("net/mlx5: fix build on Arm")
Cc: stable@dpdk.org
Signed-off-by: Michael Baum <michaelba@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>