In callout_stop(), do not forget to initialize not_running variable.

Thanks to hselasky for noticing that.

Differential Revision:	https://reviews.freebsd.org/D3078 (Updated)
Submitted by:		hselasky
Pointy hat to:		jch

sys/kern/kern_timeout.c

@@ -1385,7 +1385,8 @@ again:
 		 * and indeed impossible to stop then return 0.
 		 */
 		not_running = !(cc_exec_curr(cc, direct) == c);
-	}
+	} else
+		not_running = 1;
 
 	CC_UNLOCK(cc);
 	return (not_running);