selftests/bpf: Test sysctl_get_current_value helper

Test sysctl_get_current_value on sysctl read and write, buffers with
enough space and too small buffers to get E2BIG and truncated result,
etc.

  # ./test_sysctl
  ...
  Test case: sysctl_get_current_value sysctl:read ok, gt .. [PASS]
  Test case: sysctl_get_current_value sysctl:read ok, eq .. [PASS]
  Test case: sysctl_get_current_value sysctl:read E2BIG truncated ..  [PASS]
  Test case: sysctl_get_current_value sysctl:read EINVAL .. [PASS]
  Test case: sysctl_get_current_value sysctl:write ok .. [PASS]
  Summary: 16 PASSED, 0 FAILED

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>