init: handle ubi/mtd root mounting like all other root types

Assign a Root_Generic magic value for UBI/MTD root and handle the root
mounting in mount_root like all other root types. Besides making the
code more clear this also means that UBI/MTD root can be used together
with an initrd (not that anyone should care).

Also factor parsing of the root name into a helper now that it can
be easily done and will get more complicated with subsequent patches.

Signed-off-by: Christoph Hellwig <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

include/linux/root_dev.h

···
 enum {
 	Root_NFS = MKDEV(UNNAMED_MAJOR, 255),
 	Root_CIFS = MKDEV(UNNAMED_MAJOR, 254),
+	Root_Generic = MKDEV(UNNAMED_MAJOR, 253),
 	Root_RAM0 = MKDEV(RAMDISK_MAJOR, 0),
 };
 

init/do_mounts.c

···
 	case Root_CIFS:
 		mount_cifs_root();
 		break;
+	case Root_Generic:
+		mount_root_generic(root_device_name, root_device_name,
+				   root_mountflags);
+		break;
 	case 0:
 		if (root_device_name && root_fs_names &&
 		    mount_nodev_root(root_device_name) == 0)
···
 		mount_block_root(root_device_name);
 		break;
 	}
+}
+
+static dev_t __init parse_root_device(char *root_device_name)
+{
+	if (!strncmp(root_device_name, "mtd", 3) ||
+	    !strncmp(root_device_name, "ubi", 3))
+		return Root_Generic;
+	return name_to_dev_t(root_device_name);
 }
 
 /*
···
 
 	md_run_setup();
 
-	if (saved_root_name[0]) {
-		if (!strncmp(saved_root_name, "mtd", 3) ||
-		    !strncmp(saved_root_name, "ubi", 3)) {
-			mount_root_generic(saved_root_name, saved_root_name,
-					   root_mountflags);
-			goto out;
-		}
-		ROOT_DEV = name_to_dev_t(saved_root_name);
-	}
+	if (saved_root_name[0])
+		ROOT_DEV = parse_root_device(saved_root_name);
 
 	if (initrd_load(saved_root_name))
 		goto out;