Change proc state under held spinlock

kernel/proc/proc.c

@@ -239,7 +239,9 @@ void proc_sched(void *cpustate) {
 }
 
 void proc_kill(Proc *proc) {
+  spinlock_acquire(&PROCS.spinlock);
   proc->state = PROC_ZOMBIE;
+  spinlock_release(&PROCS.spinlock);
 }
 
 void proc_killself(void) {