x86: not clear empty_zero_page again

empty_zero_page is in .bss section, and it is cleared in clear_bss by
x86_64_start_kernel(). So don't clear that again in mem_init

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>