kcov: fix data corruption and race conditions on PREEMPT_RT

Summary

In the Linux kernel, the following vulnerability has been resolved: kcov: fix data corruption and race conditions on PREEMPT_RT syzbot is reporting KCOV state corruption on PREEMPT_RT kernels, for the temporary storage used for saving/restoring remote KCOV state is currently allocated as the per-CPU area. On PREEMPT_RT kernels, softirq handlers run as preemptible task threads (e.g., ksoftirqd). If a softirq context preempts a task running a remote KCOV session, it safely saves the task's state into the per-CPU area. However, if that softirq thread is subsequently preempted by a higher- priority softirq thread on the same CPU, the second softirq will overwrite the same per-CPU area, permanently destroying the original task's KCOV state. Fix this data corruption by moving the temporary storage from the per-CPU area to the per-thread area. Since each softirq thread now owns its own task context, nested softirq preemption no longer causes data overwrites. Note that while the temporary storage is now on a per-thread basis, the per-CPU kcov_percpu_data.lock must be retained, for we need to ensure that kcov_remote_start() and kcov_remote_stop() operate atomically without racing against asynchronous interrupts that manipulate the current task's KCOV state. It is likely that GFP_KERNEL allocation by vmalloc_node() in kcov_init() has already called panic() before returning NULL, for there will be no OOM-killable userspace processes when __init function of built-in module runs. But this patch also fixes crashing the kernel when vmalloc_node() in kcov_init() returned NULL, for kcov_init() left per-CPU irq_area == NULL but kcov_remote_start() depends on per-CPU irq_area != NULL, resulting in (1) doing vmalloc() in kcov_remote_start() despite !in_task() context (2) out-of-array-bounds access if (1) succeeded but kcov->remote_size < CONFIG_KCOV_IRQ_AREA_SIZE (3) always leak memory allocated by (1), eventually killing all OOM-killable userspace processes problems.