selftests/bpf: convert bpf tunnel test to BPF_F_ADJ_ROOM_FIXED_GSO

Lower route MTU to ensure packets fit in device MTU after encap, then
skip the gso_size changes.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>