LU-5814 obd: rename obd_unpackmd() to md_unpackmd()

LU-5814 obd: rename obd_unpackmd() to md_unpackmd()

obd_unpackmd() is only implemented by LMV so move it from OBD
operations to OBD MD operations and update the prototype to refectl
the actual usage. Remove the unused function obd_free_memmd().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I0cbbb003a94f1e5a04ba24c2a49223ff8fa16da9
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-on: http://review.whamcloud.com/13737
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>