perf tools: Simplify debugfs mountpoint handling code

We don't need to have two PATH_MAX char sized arrays holding it, just
one in util/debugfs.c will do.

Also rename debugfs_path to tracing_events_path, as it is not the path
to debugfs, that is debugfs_mountpoint. Both are now accessible.

This will allow accessing this code in the perf python binding without
having to drag in perf.c and util/parse-events.c.

The defaults for these variables are the canonical "/sys/kernel/debug"
and "/sys/kernel/debug/tracing/events/", removing the need for simple
tools to call debugfs_mount(NULL).

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-ug9jvtjrsqbluuhqqxpvg30f@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-test.c

@@ -7,6 +7,7 @@
 
 #include "util/cache.h"
 #include "util/debug.h"
+#include "util/debugfs.h"
 #include "util/evlist.h"
 #include "util/parse-options.h"
 #include "util/parse-events.h"
@@ -247,7 +248,7 @@ static int trace_event__id(const char *evname)
 
 	if (asprintf(&filename,
 		     "%s/syscalls/%s/id",
-		     debugfs_path, evname) < 0)
+		     tracing_events_path, evname) < 0)
 		return -1;
 
 	fd = open(filename, O_RDONLY);