loop: Use pr_warn_once() for loop_control_remove() warning

kernel test robot reported that RCU stall via printk() flooding is
possible [1] when stress testing.

Link: https://lkml.kernel.org/r/20211129073709.GA18483@xsang-OptiPlex-9020 [1]
Reported-by: kernel test robot <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Tetsuo Handa <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Tetsuo Handa

drivers/block/loop.c

@@ -2103,7 +2103,7 @@ static int loop_control_remove(int idx)
 	int ret;
 
 	if (idx < 0) {
-		pr_warn("deleting an unspecified loop device is not supported.\n");
+		pr_warn_once("deleting an unspecified loop device is not supported.\n");
 		return -EINVAL;
 	}