x86_64: do not reserve ramdisk two times

ramdisk is reserved via reserve_early in x86_64_start_kernel,
later early_res_to_bootmem() will convert to reservation in bootmem.

so don't need to reserve that again.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>