LU-8066 obd: Generic helpers for sysfs

LU-8066 obd: Generic helpers for sysfs

Add generic helpers to allow displaying of lustre-specific values
in /sys/fs/lustre

except devices, for now.

Linux-commit : 5e66f70e963e4ba2266071343f5a153c7db4327b

Change-Id: Idfbf419d8218546463b993a8031ef00609f8dce9
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/22209
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>