x86: make "memtest" like "memtest=17"

Impact: make boot command line "memtest" do one loop by default

So don't need to guess many patterns in one loop.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <49B10532.3020105@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>