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

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: 0c1228486befa3d6 ("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>