tools/testing/selftests/sysctl: Add pre-check to the value of writes_strict

Sysctl test will fail in some items if the value of /proc/sys/kernel
/sysctrl_writes_strict is 0 as the default value in kernel older than v4.5.

Make this test more robus and compatible with older kernel by checking and
update writes_strict value and restore it when test is done.

Signed-off-by: Orson Zhai <orson.zhai@linaro.org>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Tested-by: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>