tools lib fs debugfs: Introduce debugfs__strerror_open_tp

There will be other cases where not just a tracepoint event is being
opened below the debugfs mountpoint, but it is rather common, so provide
one helper for that.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-q6e6zct49ql6nbcw8kkg0lbj@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>