libperf: Add a perf_cpu_map__set_nr() available as an internal function for tools/perf to use

We'll need to reference count check 'struct perf_cpu_map', so wrap
accesses to its internal state to allow intercepting accesses to its
instances.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Riccardo Mancini <rickyman7@gmail.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>
Link: https://lore.kernel.org/Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/perf/cpumap.c

@@ -10,6 +10,11 @@
 #include <ctype.h>
 #include <limits.h>
 
+void perf_cpu_map__set_nr(struct perf_cpu_map *map, int nr_cpus)
+{
+	map->nr = nr_cpus;
+}
+
 struct perf_cpu_map *perf_cpu_map__alloc(int nr_cpus)
 {
 	struct perf_cpu_map *cpus = malloc(sizeof(*cpus) + sizeof(struct perf_cpu) * nr_cpus);

tools/lib/perf/include/internal/cpumap.h

@@ -28,4 +28,6 @@ struct perf_cpu_map *perf_cpu_map__alloc(int nr_cpus);
 int perf_cpu_map__idx(const struct perf_cpu_map *cpus, struct perf_cpu cpu);
 bool perf_cpu_map__is_subset(const struct perf_cpu_map *a, const struct perf_cpu_map *b);
 
+void perf_cpu_map__set_nr(struct perf_cpu_map *map, int nr_cpus);
+
 #endif /* __LIBPERF_INTERNAL_CPUMAP_H */