perf libunwind: Fixup conversion perf_sample->user_regs to a pointer

The dc6d2bc ("perf sample: Make user_regs and intr_regs optional") misses
the changes to a file, resulting in this problem:

  $ make LIBUNWIND=1 -C tools/perf O=/tmp/build/perf-tools-next install-bin
  <SNIP>
    CC      /tmp/build/perf-tools-next/util/unwind-libunwind-local.o
    CC      /tmp/build/perf-tools-next/util/unwind-libunwind.o
  <SNIP>
  util/unwind-libunwind-local.c: In function ‘access_mem’:
  util/unwind-libunwind-local.c:582:56: error: ‘ui->sample->user_regs’ is a pointer; did you mean to use ‘->’?
    582 |         if (__write || !stack || !ui->sample->user_regs.regs) {
        |                                                        ^
        |                                                        ->
  util/unwind-libunwind-local.c:587:38: error: passing argument 2 of ‘perf_reg_value’ from incompatible pointer type [-Wincompatible-pointer-types]
    587 |         ret = perf_reg_value(&start, &ui->sample->user_regs,
        |                                      ^~~~~~~~~~~~~~~~~~~~~~
        |                                      |
        |                                      struct regs_dump **
<SNIP>
  ⬢ [acme@toolbox perf-tools-next]$ git bisect bad
  dc6d2bc is the first bad commit
  commit dc6d2bc (HEAD)
  Author: Ian Rogers <irogers@google.com>
  Date:   Mon Jan 13 11:43:45 2025 -0800

      perf sample: Make user_regs and intr_regs optional

Detected using:

  make -C tools/perf build-test

Fixes: dc6d2bc ("perf sample: Make user_regs and intr_regs optional")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250313033121.758978-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/util/unwind-libunwind-local.c

@@ -579,12 +579,12 @@ static int access_mem(unw_addr_space_t __maybe_unused as,
 	int ret;
 
 	/* Don't support write, probably not needed. */
-	if (__write || !stack || !ui->sample->user_regs.regs) {
+	if (__write || !stack || !ui->sample->user_regs || !ui->sample->user_regs->regs) {
 		*valp = 0;
 		return 0;
 	}
 
-	ret = perf_reg_value(&start, &ui->sample->user_regs,
+	ret = perf_reg_value(&start, perf_sample__user_regs(ui->sample),
 			     perf_arch_reg_sp(arch));
 	if (ret)
 		return ret;
@@ -628,7 +628,7 @@ static int access_reg(unw_addr_space_t __maybe_unused as,
 		return 0;
 	}
 
-	if (!ui->sample->user_regs.regs) {
+	if (!ui->sample->user_regs || !ui->sample->user_regs->regs) {
 		*valp = 0;
 		return 0;
 	}
@@ -637,7 +637,7 @@ static int access_reg(unw_addr_space_t __maybe_unused as,
 	if (id < 0)
 		return -EINVAL;
 
-	ret = perf_reg_value(&val, &ui->sample->user_regs, id);
+	ret = perf_reg_value(&val, perf_sample__user_regs(ui->sample), id);
 	if (ret) {
 		if (!ui->best_effort)
 			pr_err("unwind: can't read reg %d\n", regnum);
@@ -741,7 +741,7 @@ static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
 	unw_cursor_t c;
 	int ret, i = 0;
 
-	ret = perf_reg_value(&val, &ui->sample->user_regs,
+	ret = perf_reg_value(&val, perf_sample__user_regs(ui->sample),
 			     perf_arch_reg_ip(arch));
 	if (ret)
 		return ret;
@@ -808,7 +808,7 @@ static int _unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 		.best_effort  = best_effort
 	};
 
-	if (!data->user_regs.regs)
+	if (!data->user_regs || !data->user_regs->regs)
 		return -EINVAL;
 
 	if (max_stack <= 0)