Bug 2307147

1. Please describe the problem:
zram has a feature where it can writeback to a backing device, this is useful for people who want to use zram but still have the advantage of writeback capability of zswap. My Arch setup for example, uses a swapfile attached as a loop device (because zram backing dev only support devices rn not files), and a systemd timer unit that triggers writeback activity.

2. What is the Version-Release number of the kernel:
6.10.3-200.fc40.x86_64

3. Did it work previously in Fedora? If so, what kernel version did the issue
   *first* appear?  Old kernels are available for download at
   https://koji.fedoraproject.org/koji/packageinfo?packageID=8 :
No

4. Can you reproduce this issue? If so, please provide the steps to reproduce
   the issue below:
  - create a zram device
  - check if /sys/block/zramX/{backing_dev,writeback} exist
  - it does not = CONFIG_ZRAM_WRITEBACK is not applied

5. Does this problem occur with the latest Rawhide kernel? To install the
   Rawhide kernel, run ``sudo dnf install fedora-repos-rawhide`` followed by
   ``sudo dnf update --enablerepo=rawhide kernel``:
No

6. Are you running any modules that not shipped with directly Fedora's kernel?:
No

7. Please attach the kernel logs. You can get the complete kernel log
   for a boot with ``journalctl --no-hostname -k > dmesg.txt``. If the
   issue occurred on a previous boot, use the journalctl ``-b`` flag.
not really needed, so here's proof the kernel isn't built with the support instead
```
cat /usr/lib/modules/6.11.0-0.rc3.20240814git6b0f8db921ab.32.fc42.x86_64/config | grep CONFIG_ZRAM_WRITEBACK
# CONFIG_ZRAM_WRITEBACK is not set
```

Reproducible: Always