loop: deprecate autoloading callback loop_probe()

The 'probe' callback in __register_blkdev() is only used under the
CONFIG_BLOCK_LEGACY_AUTOLOAD deprecation guard.

The loop_probe() function is only used for that callback, so guard it
too, accordingly.

See commit fbdee71 ("block: deprecate autoloading based on dev_t").

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230720143033.841001-2-mfo@canonical.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Mauricio Faria de Oliveira

drivers/block/loop.c

@@ -2093,6 +2093,7 @@ static void loop_remove(struct loop_device *lo)
 	put_disk(lo->lo_disk);
 }
 
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
 static void loop_probe(dev_t dev)
 {
 	int idx = MINOR(dev) >> part_shift;
@@ -2101,6 +2102,9 @@ static void loop_probe(dev_t dev)
 		return;
 	loop_add(idx);
 }
+#else
+#define loop_probe NULL
+#endif /* !CONFIG_BLOCK_LEGACY_AUTOLOAD */
 
 static int loop_control_remove(int idx)
 {