x86, kexec, 64bit: Only set ident mapping for ram.

We should set mappings only for usable memory ranges under max_pfn
Otherwise causes same problem that is fixed by

	x86, mm: Only direct map addresses that are marked as E820_RAM

This patch exposes pfn_mapped array, and only sets ident mapping for ranges
in that array.

This patch relies on new kernel_ident_mapping_init that could handle existing
pgd/pud between different calls.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-25-git-send-email-yinghai@kernel.org
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>