selftests: net: gro fwd: update vxlan GRO test expectations

UDP tunnel packets can't be GRO in-between their endpoints as this
causes different issues. The UDP GRO fwd vxlan tests were relying on
this and their expectations have to be fixed.

We keep both vxlan tests and expected no GRO from happening. The vxlan
UDP GRO bench test was removed as it's not providing any valuable
information now.

Fixes: a062260a ("selftests: net: add UDP GRO forwarding self-tests")
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>