Adding procfs support into fs class.

The interface function:
  const char *procfs__mountpoint(void);

provides existing mountpoint path for procfs.
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1383660887-1734-3-git-send-email-jolsa@redhat.com
[ Fixup namespace ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>