powerpc: Fix ppc32 mm_struct CPU tracking in SMP

The 32-bit hash code didn't need it so far so we don't update
mm->cpu_vm_mask on context switch.  This however will break when we
merge the RCU based page table freeing patch and other upcoming 32-bit
embedded SMP work, so this adds the update.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>