libperf: Rename arguments of perf_thread_map APIs

libperf: Rename arguments of perf_thread_map APIs

The "int thread" input arguments of some perf_thead_map APIs are index
of the thread in the thread map.

In order to avoid confusion and to make the APIs consistent with
perf_cpu_map APIs, those arguments are renamed to "int idx".

Suggested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20220221102612.43879-1-tz.stoyanov@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>