ovl: fix random return value on mount

commit 8fc646b4 upstream.

On failure to prepare_creds(), mount fails with a random
return value, as err was last set to an integer cast of
a valid lower mnt pointer or set to 0 if inodes index feature
is enabled.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3fe6e52f ("ovl: override creds with the ones from ...")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/overlayfs/super.c

@@ -1310,6 +1310,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	else
 		sb->s_d_op = &ovl_dentry_operations;
 
+	err = -ENOMEM;
 	ufs->creator_cred = cred = prepare_creds();
 	if (!cred)
 		goto out_put_lower_mnt;