net: expand skb_segment unit test with frag_list coverage

Expand the test with these variants that use skb frag_list:

- GSO_TEST_FRAG_LIST:             frag_skb length is gso_size
- GSO_TEST_FRAG_LIST_PURE:        same, data exclusively in frag skbs
- GSO_TEST_FRAG_LIST_NON_UNIFORM: frag_skb length may vary
- GSO_TEST_GSO_BY_FRAGS:          frag_skb length defines gso_size,
                                  i.e., segs may have varying sizes.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>