LU-8592 mdt: hold mdt_device::mdt_md_root until service stop

LU-8592 mdt: hold mdt_device::mdt_md_root until service stop

Otherwise, if someone is using such object, it may trigger
object reference ASSERTION(atomic_read(&o->lo_header->loh_ref) > 0).

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ifd7adbce9a68da537f592c64117f4ecafb0a9ec4
Reviewed-on: http://review.whamcloud.com/22438
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>