kernel_physical_mapping_init() could be called in memory hotplug path.

[ Impact: fix potential crash with memory hotplug ]
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
LKML-Reference: <20090612045752.GA827@sli10-desk.sh.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>