bpf: sockmap sample, add data verification option

To verify data is not being dropped or corrupted this adds an option
to verify test-patterns on recv.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>