use CONFIG_MEMTEST only. if it is set, will have memtest=0 (disabled)

need to have memtest=4 in command line to test more patterns.
Signed-off-by: NYinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>