btrfs: mount failure return value fix

I happened to pass swap partition as root partition in cmdline,
then kernel panic and tell me about "Cannot open root device".
It is not correct, in fact it is a fs type mismatch instead of 'no device'.

Eventually I found btrfs mounting failed with -EIO, it should be -EINVAL.
The logic in init/do_mounts.c:
        for (p = fs_names; *p; p += strlen(p)+1) {
                int err = do_mount_root(name, p, flags, root_mount_data);
                switch (err) {
                        case 0:
                                goto out;
                        case -EACCES:
                                flags |= MS_RDONLY;
                                goto retry;
                        case -EINVAL:
                                continue;
                }
		print "Cannot open root device"
		panic
	}
SO fs type after btrfs will have no chance to mount

Here fix the return value as -EINVAL
Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -1713,8 +1713,10 @@ struct btrfs_root *open_ctree(struct super_block *sb,
 		     fs_info, BTRFS_ROOT_TREE_OBJECTID);
 
 	bh = btrfs_read_dev_super(fs_devices->latest_bdev);
-	if (!bh)
+	if (!bh) {
+		err = -EINVAL;
 		goto fail_iput;
+	}
 
 	memcpy(&fs_info->super_copy, bh->b_data, sizeof(fs_info->super_copy));
 	memcpy(&fs_info->super_for_commit, &fs_info->super_copy,

fs/btrfs/volumes.c

@@ -600,8 +600,10 @@ static int __btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 		set_blocksize(bdev, 4096);
 
 		bh = btrfs_read_dev_super(bdev);
-		if (!bh)
+		if (!bh) {
+			ret = -EINVAL;
 			goto error_close;
+		}
 
 		disk_super = (struct btrfs_super_block *)bh->b_data;
 		devid = btrfs_stack_device_id(&disk_super->dev_item);
@@ -702,7 +704,7 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 		goto error_close;
 	bh = btrfs_read_dev_super(bdev);
 	if (!bh) {
-		ret = -EIO;
+		ret = -EINVAL;
 		goto error_close;
 	}
 	disk_super = (struct btrfs_super_block *)bh->b_data;
@@ -1302,7 +1304,7 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
 		set_blocksize(bdev, 4096);
 		bh = btrfs_read_dev_super(bdev);
 		if (!bh) {
-			ret = -EIO;
+			ret = -EINVAL;
 			goto error_close;
 		}
 		disk_super = (struct btrfs_super_block *)bh->b_data;