CVE-2024-26975 powercap: intel_rapl: Fix a NULL pointer dereference

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

powercap: intel_rapl: Fix a NULL pointer dereference

A NULL pointer dereference is triggered when probing the MMIO RAPL
driver on platforms with CPU ID not listed in intel_rapl_common CPU
model list.

This is because the intel_rapl_common module still probes on such
platforms even if ‘defaults_msr’ is not set after commit 1488ac990ac8
(“powercap: intel_rapl: Allow probing without CPUID match”). Thus the
MMIO RAPL rp->priv->defaults is NULL when registering to RAPL framework.

Fix the problem by adding sanity check to ensure rp->priv->rapl_defaults
is always valid.