proc: test "Kthread:" field

proc: test "Kthread:" field

/proc/${pid}/status got Kthread field recently.

Test that userspace program is not reported as kernel thread.

Test that kernel thread is reported as kernel thread.
Use kthreadd with pid 2 for this.

Link: https://lkml.kernel.org/r/818c4c41-8668-4566-97a9-7254abf819ee@p183
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Chunguang Wu <fullspring2018@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>