perf lock contention: Change stack_id type to s32

The bpf_get_stackid() helper returns a signed type to check whether it
failed to get a stacktrace or not.  But it saved the result in u32 and
checked if the value is negative.

      376         if (needs_callstack) {
      377                 pelem->stack_id = bpf_get_stackid(ctx, &stacks,
      378                                                   BPF_F_FAST_STACK_CMP | stack_skip);
  --> 379                 if (pelem->stack_id < 0)

  ./tools/perf/util/bpf_skel/lock_contention.bpf.c:379 contention_begin()
  warn: unsigned 'pelem->stack_id' is never less than zero.

Let's change the type to s32 instead.

Fixes: 6d499a6b ("perf lock: Print the number of lost entries for BPF")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240812172533.2015291-1-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/bpf_skel/lock_data.h

@@ -7,11 +7,11 @@ struct tstamp_data {
 	u64 timestamp;
 	u64 lock;
 	u32 flags;
-	u32 stack_id;
+	s32 stack_id;
 };
 
 struct contention_key {
-	u32 stack_id;
+	s32 stack_id;
 	u32 pid;
 	u64 lock_addr_or_cgroup;
 };

tools/perf/util/bpf_skel/vmlinux/vmlinux.h

@@ -15,6 +15,7 @@
 
 typedef __u8 u8;
 typedef __u32 u32;
+typedef __s32 s32;
 typedef __u64 u64;
 typedef __s64 s64;