CVE-2026-98937: perf/core: Check kernel access when kernel callchains are requested

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

perf/core: Check kernel access when kernel callchains are requested

perfeventopen() currently gates perfallowkernel() only on !attr.excludekernel.

However, users can still request kernel callchain collection with attr.excludecallchainkernel == 0 even when attr.excludekernel == 1. That still requires kernel profiling privilege, but the existing check does not enforce it.

Update the permission check to call perfallowkernel() when either kernel sampling is requested or kernel callchains are requested.

This keeps permission checks aligned with requested data and prevents unprivileged use of kernel callchain capture.