loop: don't allow deleting an unspecified loop device

Passing a negative index to loop_lookup while return any unbound device.
Doing that for a delete does not make much sense, so add check to
explicitly reject that case.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210623145908.92973-8-hch@lst.deSigned-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -2404,6 +2404,11 @@ static int loop_control_remove(int idx)
 {
 	struct loop_device *lo;
 	int ret;
+
+	if (idx < 0) {
+		pr_warn("deleting an unspecified loop device is not supported.\n");
+		return -EINVAL;
+	}
 		
 	ret = mutex_lock_killable(&loop_ctl_mutex);
 	if (ret)