MRP-1863 obdclass: don't free obddevice before selfexport

selfexport don't hold an reference now, so we may free obddevice
before selfexport, but export freeding need access to the obd device.

Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Change-Id: Ic5386b43feba079ab9402da0552d75995b7a3bf8
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/2236
Tested-by: Jenkins
Tested-by: Elena Gryaznova <elena_gryaznova@xyratex.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>