selftests: mirror_gre_lag_lacp: Check counters at tunnel

The test works by sending packets through a tunnel, whence they are
forwarded to a LAG. One of the LAG children is removed from the LAG prior
to the exercise, and the test then counts how many packets pass through the
other one. The issue with this is that it counts all packets, not just the
encapsulated ones.

So instead add a second gretap endpoint to receive the sent packets, and
check reception counters there.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>