selftest/bpf: Fix IPV6FR handling in flow dissector

From second fragment on, IPV6FR program must stop the dissection of IPV6
fragmented packet. This is the same approach used for IPV4 fragmentation.
This fixes the flow keys calculation for the upper-layer protocols.
Note that according to RFC8200, the first fragment packet must include
the upper-layer header.

Signed-off-by: Santucci Pierpaolo <santucci@epigenesys.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/X7JUzUj34ceE2wBm@santucci.pierpaolo