LU-7164 osc: osc_extent should hold refcount to osc_object

To avoid a race that osc_extent and osc_object destroy happens on the
same time, which causes kernel crash.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I3e3237f0d1cff4bd992bef4e4c01355a1d5c8d9f
Reviewed-on: http://review.whamcloud.com/16433
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>