It turns out that the cmpxchg16b emulation has to access vmalloced
percpu memory with interrupts disabled. If the memory has never
been touched before then the fault necessary to establish the
mapping will not to occur and the kernel will fail on boot.

Fix that by reusing the CONFIG_PREEMPT code that writes the
cpu number into a field on every cpu. Writing to the per cpu
area before causes the mapping to be established before we get
to a cmpxchg16b emulation.
Tested-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NChristoph Lameter <cl@linux.com>
Signed-off-by: NPekka Enberg <penberg@kernel.org>