x86, 64bit, mm: Add generic kernel/ident mapping helper

It is simple version for kernel_physical_mapping_init.
it will work to build one page table that will be used later.

Use mapping_info to control
        1. alloc_pg_page method
        2. if PMD is EXEC,
        3. if pgd is with kernel low mapping or ident mapping.

Will use to replace some local versions in kexec, hibernation and etc.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-8-git-send-email-yinghai@kernel.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>