perf/x86/rapl: Make perf_probe_msr() more robust and flexible

This patch modifies perf_probe_msr() by allowing passing of
struct perf_msr array where some entries are not populated, i.e.,
they have either an msr address of 0 or no attribute_group pointer.
This helps with certain call paths, e.g., RAPL.

In case the grp is NULL, the default sysfs visibility rule
applies which is to make the group visible. Without the patch,
you would get a kernel crash with a NULL group.

Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200527224659.206129-5-eranian@google.com