x86, mm: use pfn_range_is_mapped() with reserve_initrd

We are going to map ram only, so under max_low_pfn_mapped,
between 4g and max_pfn_mapped does not mean mapped at all.

Use pfn_range_is_mapped() to find out if range is mapped for initrd.

That could happen bootloader put initrd in range but user could
use memmap to carve some of range out.

Also during copying need to use early_memmap to map original initrd
for accessing.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-15-git-send-email-yinghai@kernel.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>