selftests/bpf: Test bpf_sysctl_{get,set}_new_value helpers

Test that new value provided by user space on sysctl write can be read
by bpf_sysctl_get_new_value and overridden by bpf_sysctl_set_new_value.

  # ./test_sysctl
  ...
  Test case: sysctl_get_new_value sysctl:read EINVAL .. [PASS]
  Test case: sysctl_get_new_value sysctl:write ok .. [PASS]
  Test case: sysctl_get_new_value sysctl:write ok long .. [PASS]
  Test case: sysctl_get_new_value sysctl:write E2BIG .. [PASS]
  Test case: sysctl_set_new_value sysctl:read EINVAL .. [PASS]
  Test case: sysctl_set_new_value sysctl:write ok .. [PASS]
  Summary: 22 PASSED, 0 FAILED

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