perf lock contention: Rework offset calculation with BPF CO-RE

It seems BPF CO-RE reloc doesn't work well with the pattern that gets
the field-offset only.  Use offsetof() to make it explicit so that
the compiler would generate the correct code.

Fixes: 0c12284 ("perf lock contention: Support pre-5.14 kernels")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Hao Luo <haoluo@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <song@kernel.org>
Cc: bpf@vger.kernel.org
Co-developed-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Link: https://lore.kernel.org/r/20230427234833.1576130-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: namhyung

tools/perf/util/bpf_skel/lock_contention.bpf.c

@@ -429,21 +429,21 @@ struct rq___new {
 SEC("raw_tp/bpf_test_finish")
 int BPF_PROG(collect_lock_syms)
 {
-	__u64 lock_addr;
+	__u64 lock_addr, lock_off;
 	__u32 lock_flag;
 
+	if (bpf_core_field_exists(struct rq___new, __lock))
+		lock_off = offsetof(struct rq___new, __lock);
+	else
+		lock_off = offsetof(struct rq___old, lock);
+
 	for (int i = 0; i < MAX_CPUS; i++) {
 		struct rq *rq = bpf_per_cpu_ptr(&runqueues, i);
-		struct rq___new *rq_new = (void *)rq;
-		struct rq___old *rq_old = (void *)rq;
 
 		if (rq == NULL)
 			break;
 
-		if (bpf_core_field_exists(rq_new->__lock))
-			lock_addr = (__u64)&rq_new->__lock;
-		else
-			lock_addr = (__u64)&rq_old->lock;
+		lock_addr = (__u64)(void *)rq + lock_off;
 		lock_flag = LOCK_CLASS_RQLOCK;
 		bpf_map_update_elem(&lock_syms, &lock_addr, &lock_flag, BPF_ANY);
 	}