perf maps: Use a pointer for kmaps

perf maps: Use a pointer for kmaps

struct maps is reference counted, using a pointer is more idiomatic.

Committer notes:

Delay:

   maps = machine__kernel_maps(&vmlinux);

To after:

  machine__init(&vmlinux, "", HOST_KERNEL_ID);

To avoid this on f34:

  In file included from /var/home/acme/git/perf/tools/perf/util/build-id.h:10,
                   from /var/home/acme/git/perf/tools/perf/util/dso.h:13,
                   from tests/vmlinux-kallsyms.c:8:
  In function ‘machine__kernel_maps’,
      inlined from ‘test__vmlinux_matches_kallsyms’ at tests/vmlinux-kallsyms.c:122:22:
  /var/home/acme/git/perf/tools/perf/util/machine.h:86:23: error: ‘vmlinux.kmaps’ is used uninitialized [-Werror=uninitialized]
     86 |         return machine->kmaps;
        |                ~~~~~~~^~~~~~~
  tests/vmlinux-kallsyms.c: In function ‘test__vmlinux_matches_kallsyms’:
  tests/vmlinux-kallsyms.c:121:34: note: ‘vmlinux’ declared here
    121 |         struct machine kallsyms, vmlinux;
        |                                  ^~~~~~~
  cc1: all warnings being treated as errors

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: André Almeida <andrealmeid@collabora.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Hao Luo <haoluo@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.garry@huawei.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Miaoqian Lin <linmq006@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Riccardo Mancini <rickyman7@gmail.com>
Cc: Shunsuke Nakamura <nakamura.shun@fujitsu.com>
Cc: Song Liu <song@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Yury Norov <yury.norov@gmail.com>
Link: http://lore.kernel.org/lkml/20220211103415.2737789-6-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>