LU-8821 mdt: avoid double find in mdt_path_current()

LU-8821 mdt: avoid double find in mdt_path_current()

In mdt_path_current() avoid finding the object we are already holding
a reference to.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Iae4796047d2c5b02989d29baf2e7620545f7e45c
Reviewed-on: https://review.whamcloud.com/23701
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>