Mark the thread pointer used during an adaptive spin volatile so that the

compiler doesn't decide to cache td_state.  Cachine the state would cause
the spinning thread to not notice when the owning thread stopped executing
(if it was preempted for example) which could result in livelock.

sys/kern/kern_mutex.c

@@ -449,7 +449,7 @@ _mtx_lock_sleep(struct mtx *m, uintptr_t tid, int opts, const char *file,
     int line)
 {
 #if defined(SMP) && !defined(NO_ADAPTIVE_MUTEXES)
-	struct thread *owner;
+	volatile struct thread *owner;
 #endif
 	uintptr_t v;
 #ifdef KTR