Fix regression introduced by r302350. The change of return value for a

callout that wasn't scheduled at all was unintentional and yielded in
several panics.

PR:		210884

sys/kern/kern_timeout.c

@@ -1381,7 +1381,7 @@ _callout_stop_safe(struct callout *c, int flags, void (*drain)(void *))
 		CTR3(KTR_CALLOUT, "failed to stop %p func %p arg %p",
 		    c, c->c_func, c->c_arg);
 		CC_UNLOCK(cc);
-		return (cancelled);
+		return (0);
 	}
 
 	c->c_iflags &= ~CALLOUT_PENDING;