net/mlx5e: Fix features validation check for tunneled UDP (non-VXLAN) packets

Move the vxlan_features_check() call to after we verified the packet is
a tunneled VXLAN packet.

Without this, tunneled UDP non-VXLAN packets (for ex. GENENVE) might
wrongly not get offloaded.
In some cases, it worked by chance as GENEVE header is the same size as
VXLAN, but it is obviously incorrect.

Fixes: e3cfc7e6 ("net/mlx5e: TX, Add geneve tunnel stateless offload support")
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>