LU-5928 obdclass: fix export of nodemap to procfs

LU-5928 obdclass: fix export of nodemap to procfs

Rewrite export code of nodemaps to use nid_stats instead of
exp directly like is done for other members.

Clean up redundant code.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: Ib3ddeabcb1c132eb1b91a0cf9583cc0f74a61e5d
Reviewed-on: http://review.whamcloud.com/13175
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>