staging: erofs: drop multiref support temporarily

commit e5e3abbadf0dbd1068f64f8abe70401c5a178180 upstream.

Multiref support means that a compressed page could have
more than one reference, which is designed for on-disk data
deduplication. However, mkfs doesn't support this mode
at this moment, and the kernel implementation is also broken.

Let's drop multiref support. If it is fully implemented
in the future, it can be reverted later.

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