init: factor the root_wait logic in prepare_namespace into a helper

The root_wait logic is a bit obsfucated right now.  Expand it and move it
into a helper.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-12-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

init/do_mounts.c

@@ -606,6 +606,26 @@ void __init mount_root(char *root_device_name)
 	}
 }
 
+/* wait for any asynchronous scanning to complete */
+static void __init wait_for_root(char *root_device_name)
+{
+	if (ROOT_DEV != 0)
+		return;
+
+	pr_info("Waiting for root device %s...\n", root_device_name);
+
+	for (;;) {
+		if (driver_probe_done()) {
+			ROOT_DEV = name_to_dev_t(root_device_name);
+			if (ROOT_DEV)
+				break;
+		}
+		msleep(5);
+	}
+	async_synchronize_full();
+
+}
+
 static dev_t __init parse_root_device(char *root_device_name)
 {
 	if (!strncmp(root_device_name, "mtd", 3) ||
@@ -642,16 +662,8 @@ void __init prepare_namespace(void)
 	if (initrd_load(saved_root_name))
 		goto out;
 
-	/* wait for any asynchronous scanning to complete */
-	if ((ROOT_DEV == 0) && root_wait) {
-		printk(KERN_INFO "Waiting for root device %s...\n",
-			saved_root_name);
-		while (!driver_probe_done() ||
-			(ROOT_DEV = name_to_dev_t(saved_root_name)) == 0)
-			msleep(5);
-		async_synchronize_full();
-	}
-
+	if (root_wait)
+		wait_for_root(saved_root_name);
 	mount_root(saved_root_name);
 out:
 	devtmpfs_mount();