Out of perf_evsel__intval(), that requires passing the variable name,
that will then be searched in the list of tracepoint variables for the
given evsel.

In cases such as syscall file descriptor ("fd") tracking, this is
wasteful, we need just to use perf_evsel__field() and cache the
format_field.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-r6f89jx9j5nkx037d0naviqy@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>