page-types.c doesn't supply a way to specify the debugfs path and the
original debugfs path is not usual on most machines.  This patch supplies
a way to auto mount debugfs if needed.

This patch is heavily inspired by tools/perf/utils/debugfs.c

[akpm@linux-foundation.org: make functions static]
[akpm@linux-foundation.org: fix debugfs_mount() signature]
Signed-off-by: NChen Gong <gong.chen@linux.intel.com>
Reviewed-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>