perf/x86/rapl: Annotate 'struct rapl_pmus' with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS=y (for
array indexing) and CONFIG_FORTIFY_SOURCE=y (for strcpy/memcpy-family
functions).

Found with Coccinelle:

  https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]

Add __counted_by for 'struct rapl_pmus'.

No change in functionality intended.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20231006201754.work.473-kees@kernel.org

arch/x86/events/rapl.c

@@ -115,7 +115,7 @@ struct rapl_pmu {
 struct rapl_pmus {
 	struct pmu		pmu;
 	unsigned int		maxdie;
-	struct rapl_pmu		*pmus[];
+	struct rapl_pmu		*pmus[] __counted_by(maxdie);
 };
 
 enum rapl_unit_quirk {