CVE-2026-89835: f2fs: avoid NULL checkpoint thread access in sysfs

In the Linux kernel, the following vulnerability has been resolved:

f2fs: avoid NULL checkpoint thread access in sysfs

checkpointmerge can be enabled even when no checkpoint merge thread is running. A read-only mount is one case: f2fs does not start f2fsissueckpt there, but ckptthreadioprio is still writable through sysfs.

The ckptthreadioprio store path updates the saved ioprio value and, when checkpointmerge is enabled, calls settaskioprio() for the checkpoint thread. If cprc->f2fsissueckpt is NULL, that dereferences a NULL task pointer.

Protect ckptthreadioprio sysfs writes with sumount as well, so the checkpoint thread cannot disappear under the store path while updating its ioprio.