Allow holding to be called with interrupts enabled (for Mark Morrissey).

spinlock.c

@@ -89,7 +89,11 @@ getcallerpcs(void *v, uint pcs[])
 int
 holding(struct spinlock *lock)
 {
-  return lock->locked && lock->cpu == mycpu();
+  int r;
+  pushcli();
+  r = lock->locked && lock->cpu == mycpu();
+  popcli();
+  return r;
 }