bpf: sockmap sample, add sendfile test

To exercise TX ULP sendpage implementation we need a test that does
a sendfile. Add sendfile test option here.

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>