x86, kexec: x86_64: add identity map for pages at image->start

Impact: Fix corner case that cannot yet occur

image->start may be outside of 0 ~ max_pfn, for example when jumping
back to original kernel from kexeced kenrel. This patch add identity
map for pages at image->start.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>