Checkpoint page-table version for SMP

Includes code for TLB shootdown (which actually seems unnecessary for xv6)

proc.c

@@ -242,6 +242,7 @@ sched(void)
     panic("sched running");
   if(readeflags()&FL_IF)
     panic("sched interruptible");
+  loadkvm();    // Switch to the kernel page table
   intena = cpu->intena;
   swtch(&proc->context, cpu->scheduler);
   cpu->intena = intena;