If destroying a spinlock, make sure that it is exited properly.

Submitted by: jhb
MFC After: 3 days

sys/kern/kern_mutex.c

@@ -883,6 +883,10 @@ mtx_destroy(struct mtx *m)
 	else {
 		MPASS((m->mtx_lock & (MTX_RECURSED|MTX_CONTESTED)) == 0);
 
+		/* Perform the non-mtx related part of mtx_unlock_spin(). */
+		if (LO_CLASSINDEX(&m->mtx_object) == LOCK_CLASS_SPIN_MUTEX)
+			spinlock_exit();
+
 		/* Tell witness this isn't locked to make it happy. */
 		WITNESS_UNLOCK(&m->mtx_object, LOP_EXCLUSIVE, __FILE__,
 		    __LINE__);