tools lib api fs: Add tracefs mount helper functions

Since tracefs will now hold the event directory for perf, and even
though by default, debugfs still mounts tracefs on the debugfs/tracing
directory, the system admin may now choose to not mount debugfs and
instead just mount tracefs instead.

Having tracefs helper functions will facilitate having perf look for
tracefs first, and then try debugfs as a fallback.

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/20150202193552.898934751@goodmis.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>