CVE-2026-98790: perf: Fix race between perf_event_exit_task() and perf_pending_task()

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

perf: Fix race between perfeventexittask() and perfpendingtask()

A race condition exists between perfeventexittask() and perfpendingtask() during beginnewexec().

During beginnewexec(), perfeventexittask() may be called, and the PFEXITING flag is not set on task. So perfsigtrap() continues to execute and triggers WARNONONCE(event->ctx->task != current).

Since both task exit and exec paths can call perfeventexittask() which sets ctx->task to TASKTOMBSTONE, fix this by explicitly checking if event->ctx->task equals TASKTOMBSTONE and dropping the redundant PFEXITING check.