CVE-2026-98536: writeback: report a Tasks-RCU quiescent state per cgwb drain pass

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

writeback: report a Tasks-RCU quiescent state per cgwb drain pass

cleanupofflinecgwbsworkfn() drains a dying cgwb by calling cleanupofflinecgwb() until it returns false, with a condresched() between passes. On a CONFIGPREEMPTION kernel that condresched() does nothing: condresched() is a plain "return 0", and under PREEMPTDYNAMIC the full and lazy modes disable it. Since commit 7dadeaa6e851 ("sched: Further restrict the preemption modes") those are the only two models on the architectures with PREEMPTLAZY support, arm64 and x86 among them, so the drain loop never reports a Tasks-RCU quiescent state.

A worker draining a cgwb with millions of attached inodes runs for minutes. On a 6.18 arm64 host in lazy mode the cgwb worker drained one dying cgroup's writeback domain for over 11 minutes. A BPF program unlink (bpftrampolineunlinkprog -> bpftrampolineupdate -> unregisterftracedirect -> ftraceshutdown -> synchronizercutasks()) waited on that grace period while holding the trampoline mutex, 42 tasks queued behind it in D state, and the hung task detector fired at 614 s and panicked the host. Any BPF or ftrace detach during a long drain inherits the drain's length.

Fix this by calling condreschedtasksrcuqs() so we do not stall out anybody who calls sycnrhonizercutasks(). We put this in a do { } while loop because if we have many small cgroups cleanupofflinecgwb() will return false and we will never call condreschedtasksrcuqs(), creating the same problem.