staging: erofs: fix potential double iput in erofs_read_super()

Some error cases like failing from d_make_root() will
cause double iput because d_make_root() also does iput
in its error path.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>