staging/lustre/llite: ensure obd is effective in onu_upcall

The watched obd device may still not setup while onu_upcall
invoked. So we need verify it in cl_ocd_update.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-on: http://review.whamcloud.com/19597
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8027


Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>