tools lib api debugfs: Add DEBUGFS_DEFAULT_PATH macro

Instead of hard coding "/sys/kernel/debug" everywhere, create a macro to
hold where the default path exists.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20150202193553.032117017@goodmis.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>