CVE-2026-99187: perf: Fix perf_pmu_migrate_context() leaving events behind

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

perf: Fix perfpmumigratecontext() leaving events behind

perfpmuremove() walks the group leaders of a CPU and PMU with perfeventgroupsforcpupmu(), removing each one as it goes. perfremovefromcontext() takes the leader out of the groups rbtree, after which rbnext() on that node returns NULL, prematurely ending the walk. Other groups stay behind in the source context.

On CPU hotplug, they are then torn down with the dying CPU. They keep running until teardown, while the PMU's interrupt and migrated events are on a new CPU, which drivers rely on never happening.

Walk ctx->eventlist instead and pick out the PMU's events by their pmuctx.