selftests/bpf: test that GSO works in lwt_ip_encap

Add a test on egress that a large TCP packet successfully goes through
the lwt+bpf encap tunnel.

Although there is no direct evidence that GSO worked, as opposed to
e.g. TCP segmentation or IP fragmentation (maybe a kernel stats counter
should be added to track the number of failed GSO attempts?), without
the previous patch in the patchset this test fails, and printk-debugging
showed that software-based GSO succeeded here (veth is not compatible with
SKB_GSO_DODGY, so GSO happens in the software stack).

Also removed an unnecessary nodad and added a missed failed flag.

Signed-off-by: Peter Oskolkov <posk@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>