test_bpf: allow tests to specify an skb fragment.

This introduce a new test->aux flag (FLAG_SKB_FRAG) to tell the
populate_skb() function to add a fragment to the test skb containing
the data specified in test->frag_data).

Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>