Enable CONFIG_BLK_DEV_UBLK and CONFIG_ZRAM_WRITEBACK in config-wsl

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**

I've been building a multi-tier swap stack on WSL2 (compressed RAM + a userspace block backend for colder pages). On the stock Microsoft kernel both of these show up as off:

```
# CONFIG_BLK_DEV_UBLK is not set
# CONFIG_ZRAM_WRITEBACK is not set
```

`CONFIG_ZRAM` itself is already `=m` and swap works fine. I/O uring is already on. NBD is there as a module too. So the missing pieces are specifically ublk and zram writeback.

Without them I have to rebuild from https://github.com/microsoft/WSL2-Linux-Kernel every time I want ublk (instead of sitting on NBD only) or attach a writeback device under zram. That works, but it's a lot of friction for something that is already upstream and only needs a config flip.

**Describe the solution you'd like**

In `Microsoft/config-wsl` (and arm64 if it applies the same way):

```
CONFIG_BLK_DEV_UBLK=m
CONFIG_ZRAM_WRITEBACK=y
```

Module for ublk so it stays out of the way until someone loads it. Writeback only does something if userspace actually sets a backing device.

**Describe alternatives you've considered**

- Custom kernel from `linux-msft-wsl-6.18.y` / `6.6.y` with those two options forced on. This works on my machine today.
- Stick to NBD only. Fine for experiments, worse latency/CPU profile than ublk for the same userspace server in my tests.

I'd rather not carry a private kernel long-term if the stock config can ship the modules.

**Additional context**

Checked against a recent `config-wsl` from the 6.18.y line. After `olddefconfig` both options stick cleanly:

- `CONFIG_BLK_DEV_UBLK=m`
- `CONFIG_ZRAM_WRITEBACK=y`

No out-of-tree modules involved — just enabling code that is already in the tree you ship.

Happy to retest on a preview kernel if that helps.

Start in Microsoft/config-wsl and check whether the arm64 configuration applies the same settings. Run olddefconfig to confirm the requested options remain enabled, then verify that CONFIG_BLK_DEV_UBLK=m and CONFIG_ZRAM_WRITEBACK=y are present in the resulting configuration.

Written by the indexing model from the issue text.