perf/amd/ibs: Avoid calling perf_allow_kernel() from the IBS NMI handler

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

perf/amd/ibs: Avoid calling perf_allow_kernel() from the IBS NMI handler

Calling perf_allow_kernel() from the NMI context is unsafe and could be fatal. Capture the permission at event-initialization time by storing it in event->hw.flags, and have the NMI handler rely on that cached flag instead of making the call directly.