If the panic thread is preempted after setting panicstr but before

setting TDF_INPANIC then it will never be rescheduled again. Wrap
setting the panic condition with the critical section.

Noted and reviewed by:	tegge
MFC after:	1 week

sys/kern/kern_shutdown.c

@@ -511,6 +511,7 @@ panic(const char *fmt, ...)
 	va_list ap;
 	static char buf[256];
 
+	critical_enter();
 #ifdef SMP
 	/*
 	 * We don't want multiple CPU's to panic at the same time, so we
@@ -569,6 +570,7 @@ panic(const char *fmt, ...)
 	/* thread_unlock(td); */
 	if (!sync_on_panic)
 		bootopt |= RB_NOSYNC;
+	critical_exit();
 	boot(bootopt);
 }