bpf: Add selftests for BPF_MODIFY_RETURN

Test for two scenarios:

  * When the fmod_ret program returns 0, the original function should
    be called along with fentry and fexit programs.
  * When the fmod_ret program returns a non-zero value, the original
    function should not be called, no side effect should be observed and
    fentry and fexit programs should be called.

The result from the kernel function call and whether a side-effect is
observed is returned via the retval attr of the BPF_PROG_TEST_RUN (bpf)
syscall.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-8-kpsingh@chromium.org