loop: reduce loop_ctl_mutex coverage in loop_exit

loop_ctl_mutex is only needed to iterate the IDR for removing the loop
devices, so reduce the coverage.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210623145908.92973-3-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -2549,13 +2549,14 @@ static int loop_exit_cb(int id, void *ptr, void *data)
 
 static void __exit loop_exit(void)
 {
-	mutex_lock(&loop_ctl_mutex);
 	unregister_blkdev(LOOP_MAJOR, "loop");
 	misc_deregister(&loop_misc);
 
+	mutex_lock(&loop_ctl_mutex);
 	idr_for_each(&loop_index_idr, &loop_exit_cb, NULL);
-	idr_destroy(&loop_index_idr);
 	mutex_unlock(&loop_ctl_mutex);
+
+	idr_destroy(&loop_index_idr);
 }
 
 module_init(loop_init);