When using for_each_event() we needlessly rebuild the whole path to
the tracepoint directory, reuse the dir_path instead, saving some cycles
and reducing the size of the next patch.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-54bcs15n0cp6gwcgpc4hptyc@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>